Q: Do Muslims and Christians worship the same God?
A: Yes. The Second Vatican Council teaches that Islam is an Abrahamic faith; that is, that like Christianity and Judaism, Islam derives its basic reason for existing from the fundamental relationship established between Abraham and God.
Paragraph 16 of The Dogmatic Constitution on the Church (Lumen gentium)* reads, in part:
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. But the plan of salvation also includes those who acknowledge the Creator. In the first place amongst these there are the Mohammedans, 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.
Catholics who oppose this teaching (i.e. who claim that Muslims worship an alien god) fail to make several distinctions proper to this teaching. . .
That Muslims worship the God the Bible does not mean that they possess the same understanding of God that Christians do. Muslims deny the Trinity, the Incarnation, and the Atonement. This is just another way of saying that Muslims aren't Christians. Jews also deny fundamental Christian beliefs, most especially that Jesus is the promised Messiah. Yet, the Church does not hesitate to teach that Jews worship the God of the Bible.
That Muslims worship the God the Bible does not mean that they possess the same understanding of God that Christians do. Muslims deny the Trinity, the Incarnation, and the Atonement. This is just another way of saying that Muslims aren't Christians. Jews also deny fundamental Christian beliefs, most especially that Jesus is the promised Messiah. Yet, the Church does not hesitate to teach that Jews worship the God of the Bible.
That Muslims accept the Bible as a deeply flawed yet nonetheless sacred book does not mean that they reject the historical origins or all the prophetic pronouncements of the Bible. Muslims claim that Jews and Christians have misinterpreted the Bible and invented several false doctrines using the Bible. Protestants make similar claims about the Catholic Church. Yet, the Church does not hesitate to teach that Protestants worship the God of the Bible.
That Muslims themselves have used the Bible to invent false doctrines does not mean that they worship a god other than the God of the Bible. The doctrinal inventions of Islam are readily recognized by the Church as false. The Church also recognizes that many of the doctrinal inventions of Protestants, Unitarians, Mormons, etc. are false. Yet, we do not claim that these groups give worship to an alien god.
Muslims honor Jesus as a prophet and offer devotion to Blessed Mary as his virginal mother. They honor the Old Testament prophets and trace their religious foundation to Abraham's covenant with God. Their essential difference with Christians is their belief that almost all of the claims made by the Church regarding the identity of the Messiah are false. So, though Muslims worship the God of the Bible, they do so in seriously theologically deficient ways.
The Church have never taught and does not teach that Islam's theology is the same as the Church's. Nor does the Church teach that Muslims achieve salvation through their Prophet or by adhering to orthodox Muslim teaching. Salvation is achieved only through Christ in his Church. What Lumen gentium teaches is that if a Muslim finds himself in heaven, he is there because of Christ. How he arrived in heaven is strictly a matter for God. The sacraments of the Church are gifts that necessarily limit creatures not the Creator. All the Church is teaching here is that God's creatures have no power to compel their Creator to include or exclude any one person or any group of people from heaven. The sacraments of the Church are the ordinary means of salvation. for believers. They are not the only means. God is perfectly free to save whomever He wishes. And we are in no position to demand that He do otherwise (cf. Dominus Iesus, n. 20-22).
UPDATE: HancAquam regular, dimbulb, sends along this little 11thc. nugget:
From Letter XXI of Pope St. Gregory VII (†1085) to the (Muslim) King of Mauritania:
"[F]or Almighty God, Who desires that all men shall be saved and that none shall perish, approves nothing more highly in us than this: that a man love his fellow man next to his God and do nothing to him which he would not that others should do to himself.
UPDATE: HancAquam regular, dimbulb, sends along this little 11thc. nugget:
From Letter XXI of Pope St. Gregory VII (†1085) to the (Muslim) King of Mauritania:
"[F]or Almighty God, Who desires that all men shall be saved and that none shall perish, approves nothing more highly in us than this: that a man love his fellow man next to his God and do nothing to him which he would not that others should do to himself.
This affection we and you owe to each other in a more peculiar way than to people of other races because we worship and confess the same God though in diverse forms and daily praise and adore Him as the creator and ruler of this world. For, in the words of the Apostle, 'He is our peace who hath made both one.'
This grace granted to you by God is admired and praised by many of the Roman nobility who have learned from us of your benevolence and high qualities.[. . .]
For God knows our true regard for you to his glory and how truly we desire your prosperity and honor, both in this life and in the life to come, and how earnestly we pray both with our lips and with our heart that God Himself, after the long journey of this life, may lead you into the bosom of the most holy patriarch Abraham."
*This document is a dogmatic constitution promulgated by a legitimately assembled ecumenical council of the Church and approved by the Holy Father. Interpretations of this document and claims about its implications are perfectly legitimate. Outright denials of the document's authority to teach the faith are not legitimate.
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