I believe they do worship the same God, as the Church acknowledges, for the following reasons.
First, I think there should be noted a difference between worshipping God in Spirit and in truth, being in the grace of the Father or having Him, etc., and worshipping God according to the virtue of religion (which is not a theological virtue, but falls under justice).
St. Thomas defines this virtue in the Summa as “to show reverence to one God under one aspect, namely, as the first principle of the creation and government of things.”
newadvent.org/summa/3081.htm
Can Muslims do this? They certainly worship “God” as First Principle and Supreme Governor of all things, but is it the same God we know? Can one acknowledge the one God without acknowledging the Trinity?
First, it needs to be pointed out that faith is required to acknowledge the Trinity. The Trinity cannot be reasoned out, as St. Vincent Ferrer explains:
St. Vincent Ferrer:
Concerning the use of the intelligence with regard to the Trinity, St. Thomas asks whether the Trinity of the Divine Persons can be known by natural reasoning. He answers: “It is impossible to attain to the knowledge of the Trinity by natural reason.” For man can obtain the knowledge of God by natural reason only from creatures. Now creatures lead us to God as effects do to their cause. Accordingly, by natural reason we can know of God that only which of necessity belongs to him as the principle of all things. Now, the creative power of God is common to the whole Trinity; and hence it belongs to the unity of the essence, and not to the distinction of the Persons. Therefore by natural reason we can know what belongs to the unity of the essence, but not what belongs to the distinction of the Persons. Whoever, then, tries to prove the Trinity of Persons by natural reason, derogates from faith.
Therefore, we can know of God, as the Principle of all things, from reason alone, apart from faith, but we can only know of the Trinity with faith since it is a revealed dogma. The First Vatican Council also defined that God can be known from natural reason alone (Dei Filius, Canon 2.1) and St. Paul says, on account of this, those who do not acknowledge God (but worship idols, are atheists, etc.) are without excuse (Rom. 1:20).
Therefore, one can acknowledge the one God and Creator of all things without having faith and acknowledging the Trinity. But do Muslims do this?
How can we say whether or not we are talking about the same thing? It is the essence of the thing that determines what it is. If we acknowledge the same essence, we acknowledge the same thing. What we can say about the essence of God is that it is the same as His existence. This is summed up as “God is” or, in His own words, “I AM” or “I AM who AM.” (Exo. 3:14)
This concept is formally referred to as the “aesity” of God. Essentially, aesity means self-existence. Aesity explains the metaphysical nature of God as a purely self-existent being that exists in complete actuality. God is not a being that is created by another god; neither does God create Himself into existence. Rather, God has always existed as an unchanging, completely actualized being. God has his Being of himself and to himself such that he is absolute being and the very definition of existence (Acts 17:22-28). Since God’s existence is the same as his essence, it follows that God is existence. (Note: this not to assert pantheism. All other beings participate in his existence on a contingency and thus do not possess the essence of God. Therefore, no other being can be said to be a god or share a part in godhead since they exist solely on a contingency.) This concept is at the root of the definition of all of God’s other perfections because if God is absolute being he must logically contain in Himself all perfections of being.
Since God’s essence is existence, if one acknowledges His essence, one can only acknowledge He who exists–it is impossible to acknowledge a completely actualized being that is not the true God. Similarly, there cannot exist two of such beings, because then neither would contain in Himself all perfections of being. The Catholic Encyclopedia article on Essence and Existence gives the Thomist position on this:
-If essence and existence were but one thing, we should be unable to conceive the one without conceiving the other. But we are as a fact able to conceive of essence by itself.
-If there be no real distinction between the two, then the essence is identical with the existence. But in God alone are these identical.
newadvent.org/cathen/05543b.htm
In other words, when essence and existence are but one thing, we cannot conceive of one without conceiving of the other. And this is the case only with God. Therefore, to conceive a being with aesity is to conceive of the one God who exists–it’s impossible to conceive of something where essence and existence are identical, but that is not God.
Since Muslims do conceive of God as being completely self-actualized as far as I can tell (see
here for example)–of being non-contingent–as having aesity, then they therefore can only be said to acknowledge the one God who exists and it is to Him that they honor as First Principle and Creator according to the virtue of religion.
I would say therefore that we know God; they know of God. We worship Him in Spirit and in Truth and serve Him in supernatural faith, they worship only in a natural way–but they do adore Him, despite their other errors.
