Does the Catholic Church teach that Christians and Muslims worship the same God? Matt Slick, President and Founder of the Christian Apologetics and Research Ministry (CARM), attempts to answer this question on his website, but he's wrong about the Catholic position. Let's have a look at his answer:

Is Allah, the God of Islam, the same as the Yahweh the God of the Bible?

No, the God of Islam is not the same as the God of Christianity. In Christianity God is a Trinity, but in Islam God is not a Trinity. It is not possible to have God be a Trinity and also not a Trinity at the same time. Furthermore, because Islam denies the Trinity (that there is one God who is comprised of three distinct persons: Father, Son, and Holy Spirit), it means that Jesus is not the incarnation of the second person of the Trinity.
  • Quran, 5:73 (Yusuf Ali), “They do blaspheme who say: Allah is one of three in a Trinity: for there is no god except One Allah. If they desist not from their word (of blasphemy), verily a grievous penalty will befall the blasphemers among them.”
So it is clear that the Quran clearly denies one of the essential Christian doctrines about God’s nature. The God of Islam and the God of Christianity are not the same.

Also, in Islam Jesus is not God in flesh (Surah 4:172, 5:73, 9:30) where in Christianity He is (John 1:1, 14, Col. 2:9). In Christianity Jesus was crucified, but in Islam He was not (Surah 4:158). This is another irreconcilable difference between Islam and Christianity regarding God and who Christ is. In addition, Islam says that the Helper (paraclete in Greek) is Muhammed where the Bible says that the Helper is the Holy Spirit (John 14:26). So, the God of Islam and the God of Christianity are not the same.

However, we also need to point out a gross error on the part of Roman Catholicism regarding this issue. The Catechism of the Catholic Church says in paragraph 841, “The Church's relationship with the Muslims. 'The plan of salvation also includes those who acknowledge the Creator, in the first place amongst whom are the Muslims; these profess to hold the faith of Abraham, and together with us they adore the one, merciful God, mankind's judge on the last day.'" The Roman Catholic church is wrong. Muslims and Christians do not worship the same God.
Are Allah and Yahweh the same God?

Slick's answer is fairly accurate, except for the last paragraph. He quotes the Catechism out of context and misreads it, obviously not understanding the Church's position on Islam. Admittedly, the paragraph is a little unclear. Because it says that Muslims "adore the one, merciful God,"  it could give the impression that Muslims can be saved by adhering to Islam. However, the original context of this statement shows that this is not at all what the Catechism says.

The following is largely based on Jimmy Akin's article The Catechism on Islam:

Lumen Gentium on the People of God

The Catechism's paragraph on Islam (CCC 841) is a quote from the Vatican II Dogmatic Constitution on the Church, Lumen Gentium (LG) 16. Chapter 2 of Lumen Gentium discusses the nature of the people of God, and how various people are related to Christ and the Church.  

To understand what the Catechism (and Lumen Gentium) really say about Islam, we have to back up to LG 13 and read through LG 17.

Lumen Gentium 13: All People Called to Belong to the New People of God

LG 13 unambiguously states that "all people are called to belong to the new people of God," the Church, which Christ established for the purpose of communicating his salvation to the world. LG 13 then notes three categories of people who are related to the Church in different ways, going from the closest to the farthest: (1) the Catholic faithful; (2) others who believe in Christ (non-Catholic Christians); and (3) the rest of mankind (non-Christians).

All men are called to be part of this catholic unity of the people of God which in promoting universal peace presages it. And there belong to or are related to it in various ways, (1) the Catholic faithful, (2) all who believe in Christ, and (3) indeed the whole of mankind, for all men are called by the grace of God to salvation.

The next three sections of LG elaborate on how these three groups are related to the Church.

Lumen Gentium 14: Catholics

LG 14 concerns itself with Catholics. It states:

 This Sacred Council wishes to turn its attention firstly to the Catholic faithful. Basing itself upon Sacred Scripture and Tradition, it teaches that the Church, now sojourning on earth as an exile, is necessary for salvation. Christ, present to us in His Body, which is the Church, is the one Mediator and the unique way of salvation. In explicit terms He Himself affirmed the necessity of faith and baptism and thereby affirmed also the necessity of the Church, for through baptism as through a door men enter the Church. Whosoever, therefore, knowing that the Catholic Church was made necessary by Christ, would refuse to enter or to remain in it, could not be saved.

LG 14 thus makes it quite clear that the only way of salvation is found in Christ through the Church, which is entered through faith and baptism. If someone knowingly and willfully rejects Christ and the Church he established, aware that Christ made it necessary for salvation, that person "could not be saved."  These strong words repudiate the idea that Islam or any other religion is as good as Christ and the Church, or could have any salvific power.

Lumen Gentium 15: Non-Catholic Christians

LG 15 then turns to non-Catholic Christians and states: 

The Church recognizes that in many ways she is linked with those who, being baptized, are honored with the name of Christian, though they do not profess the faith in its entirety [e.g. Protestants] or do not preserve unity of communion with the successor of Peter [e.g. Orthodox]. 

LG 15 says that non-Catholic Christians are "linked" to the Church in many ways (e.g. Scripture, faith in Christ, baptism, prayer), and so they are "joined to us [Catholics] in the Holy Spirit]. However, this union is partial and imperfect because non-Catholic Christians do not accept the fullness of the faith, and so the Church prays that all may be "peacefully united, in the manner determined by Christ, as one flock under one shepherd" in the Catholic Church.

Lumen Gentium 16: Non-Christians

LG 16—the source for CCC 841—now turns to the case of non-Christians. It's worth quoting this paragraph in full (with subdivisions added for clarity).

16. Finally, those who have not yet received the Gospel are related in various ways to the people of God.

(a) In the first place we must recall the people to whom the testament and the promises were given and from whom Christ was born according to the flesh. On account of their fathers this people remains most dear to God, for God does not repent of the gifts He makes nor of the calls He issues.

(b) But the plan of salvation also includes those who acknowledge the Creator. In the first place amongst these there are the Muslims, who, professing to hold the faith of Abraham, along with us adore the one and merciful God, who on the last day will judge mankind. Nor is God far distant from those who in shadows and images seek the unknown God, for it is He who gives to all men life and breath and all things, and as Saviour wills that all men be saved.

(c) Those also can attain to salvation who through no fault of their own do not know the Gospel of Christ or His Church, yet sincerely seek God and moved by grace strive by their deeds to do His will as it is known to them through the dictates of conscience. Nor does Divine Providence deny the helps necessary for salvation to those who, without blame on their part, have not yet arrived at an explicit knowledge of God and with His grace strive to live a good life. Whatever good or truth is found amongst them is looked upon by the Church as a preparation for the Gospel. She knows that it is given by Him who enlightens all men so that they may finally have life.

(d) But often men, deceived by the Evil One, have become vain in their reasonings and have exchanged the truth of God for a lie, serving the creature rather than the Creator. Or some there are who, living and dying in this world without God, are exposed to final despair.

(e) Wherefore to promote the glory of God and procure the salvation of all of these, and mindful of the command of the Lord, "Preach the Gospel to every creature", the Church fosters the missions with care and attention.

This paragraph discusses the condition of "those who have not yet received the Gospel"—those who are not of the household of faith. It considers how non-Christians are related to the people of God, the Church—in other words, non-Christians don't belong to the people of God.  Of these people, LG 16 specifically mentions the adherents of the two great monotheistic religions that are closest to Christianity, Judaism and Islam.

LG 16 begins with (a) the Jewish people, who are more closely related to the Church than any other non-Christian religion. As the Catechism states, "The Jewish faith, unlike other non-Christian religions, is already a response to God's revelation in the Old Covenant" (CCC 839).

Next, LG 16 turns to (b) Muslims. Note that LG 16 discusses how Muslims (the people) are related to God's plan of salvation; it does not say that Islam (the religion) is a valid path to salvation.

As Akin explains, the subject under discussion is not everyone who is saved but how various people are related to the Catholic Church. The Council has already stated that Christ and the Church are necessary for salvation; it now describes people who are progressively more distant from the Church. And so when we encounter the statement that "the plan of salvation also includes" these people, we should not understand it as saying that they are saved. It means that God desires their salvation and has made plans for their salvation—plans that include giving them graces that lead in the direction of salvation and the Church. But that doesn’t mean that they can be saved by being nothing more than monotheists. Within this category, Muslims today hold the first place in that they are the largest such group and have a number of commonalities with Judaism and Christianity, several of which the council goes on to note:

(i) They "profess to hold the faith of Abraham." The operative word here is "profess"—they claim to hold the faith of Abraham. In reality, their faith is an imperfect version of the faith that comes from Abraham, but they are trying to follow in the footsteps of Abraham, and the Council gives them credit for that.

(ii) "Together with us they adore the one, merciful God." For many, this statement is perplexing. However, God is aware of and acknowledges all that is good and true in the worship offered to him, however imperfect an understanding of him a worshiper may have. While Muslims, like Jews, do not accept the Trinity or Jesus' identity as Son of God, they do acknowledge that God is the only true God and that he is merciful. This means that they honor things that are true about God but have a limited understanding of him.

(iii) Muslims recognize that God is "mankind’s judge on the last day." This is another link they have to biblical faith. Muslims may have erroneous ideas about some of the things that will occur before, after, or around this event, but that much they have right.

Note that Muslims are mentioned together with "those who in shadows and images seek the unknown God," the God who "as Saviour wills that all men be saved."   While God indeed wills that Muslims and others be saved, the Council does not indicate that Islam (or any other non-Christian religion) is a path to salvation.  LG 16 goes on to say that (c) some in those religions can be saved if they do not know Christ and the Church through no fault of their own and "sincerely seek God," striving to do His will as it is known to them through their conscience.  And yet, (d) members of non-Christian religions are particularly prone to be "deceived by the Evil One," to "become vain in their reasonings," and to "exchange the truth of God for a lie."  This is hardly a ringing endorsement of Islam or of any other non-Christian religion.

Therefore, since other religions can only provide glimpses and shadows of incomplete truth, (e) the Church has the obligation to carry out her work of evangelization and proclaim Christ's saving truth to people of all religions—including Muslims—to promote the glory of God and extend Christ's salvation to them.  

Lumen Gentium 17: Proclaiming Christ to all People

The Church's missionary mandate to all people is made abundantly clear in LG 17.  The Church has the responsibility and obligation to proclaim Christ to all people:

17. As the Son was sent by the Father, so He too sent the Apostles, saying: "Go, therefore, make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things whatsoever I have commanded you. And behold I am with you all days even to the consummation of the world". The Church has received this solemn mandate of Christ to proclaim the saving truth from the apostles and must carry it out to the very ends of the earth. Wherefore she makes the words of the Apostle her own: "Woe to me, if I do not preach the Gospel", and continues unceasingly to send heralds of the Gospel until such time as the infant churches are fully established and can themselves continue the work of evangelizing. For the Church is compelled by the Holy Spirit to do her part that God's plan may be fully realized, whereby He has constituted Christ as the source of salvation for the whole world. 

The Declaration Dominus Iesus

Even though LG 17 is quite clear that Christ is the only source of salvation for all people, lingering confusion in this area was addressed in the document Dominus Iesus, which was released by the Congregation for the Doctrine of the Faith in 2000.

According to the document, "It would be contrary to the faith to consider the Church as one way of salvation alongside those constituted by the other religions, seen as complementary to the Church or substantially equivalent to her" (DI 21).

Further, "if it is true that the followers of other religions can receive divine grace, it is also certain that objectively speaking they are in a gravely deficient situation in comparison with those who, in the Church, have the fullness of the means of salvation" (DI 22).

There should be no doubt that the Church recognizes that followers of Islam have elements of truth. But while it is possible for them—as for all men—to be saved if they live up to the light God has given them, it cannot be said that Islam is a path of salvation or that Muslims do not need to encounter Jesus Christ to be saved.

Do Christians and Muslims Worship the Same God?

Now that it's clear that the Church does not consider Islam to be a valid path of salvation, we can return to our original question: do Christians and Muslims worship the same God? Jimmy Akin has also addressed this issue (the following is adapted from his article Allah = God?)

When Muslims talk to Allah, are they talking to God?

We need not be detained by the fact that the word "Allah" is not the normal English word for God. It is the normal Arabic word for God, and it is used by Arabic-speaking Christians as a designator for the true God all the time.  When they use the word "Allah", they have in mind a Trinitarian Being, the Second Person of whom became incarnate as Jesus Christ. That’s what Arabic-speaking Christians mean by "Allah."

Arabic-speaking Muslims (and other Muslims) obviously mean something different, and the question is whether their usage of the term is different enough that it would prevent prayers they address to Allah from being prayers addressed to God.

What characteristics does a Muslim typically envision Allah as having? According to Muslims, God:

  1. Is an uncreated being
  2. Is the creator of the universe
  3. Appeared to Abraham
  4. Is just
  5. Is merciful
  6. Will raise the dead
  7. Is not a Trinity
  8. Is not incarnate as Jesus of Nazareth

Characteristics 1-6 are ones that Christians agree with Muslims about. It is characteristics 7 and 8 that are the key points of disagreement. Are they sufficient to keep God from receiving Muslim prayers directed to him?

Before answering that question, note that a non-Christian Jewish person would say exactly the same list of characteristics applies to the God to whom they direct their prayers. And yet Christian tradition and the Bible itself acknowledge that Jewish individuals do worship and pray to God, even if they do not understand that he is a Trinity or that he is incarnate as Jesus of Nazareth. And so it is quite possible for Jews to talk to God (and be heard by Him), even if there are things that they don’t know about Him or even have false beliefs about Him.

But what about the prayers of Muslims? Are they addressed to the true God?  As Akin says,  it depends on the Muslim in question. Some Muslims may be so anti-Christian that they would be unwilling to talk to God–to Allah–if it turned out that he was the God of the Christians. Those Muslims would not be talking to God because there is no being that corresponds to the description "the true God who is not the God of the Christians." They would be talking to the void.

On the other hand, many Muslims may not believe that God is a Trinity or that he incarnated in Jesus of Nazareth, but they are still sincerely directing their prayers to something like the "Creator of the Universe" or "the God who appeared to Abraham" or "the one true God" or something like that. Can we really exclude the possibility that God hears the prayers of pious Muslims who pray to God as they understand Him?

This is what enables the Catechism to state that, despite substantial differences in their understanding of God, Muslims "acknowledge the Creator" and "together with us they adore the one, merciful God" (CCC 841)—even if their religious experience as Muslims is not necessarily salvific.

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