Objection 1. It would seem that Christians and Muslims do not worship the same God. For Christians firmly believe God is a Trinity, which is what Muslims emphatically deny. If they disagree on so an important doctrine about God, it seems probable that they do not believe in and worship the same God.
Objection 2. Christ said: no one knows the Son except the Father, just as no one knows the Father except the Son and those to whom the Son chooses to reveal Him (Matthew 11:27). But Muslims, since they deny the divinity of Christ, cannot have had the Son’s revelation. It follows that they do not know the Father, and since no one can worship what he does not know, Muslims do not worship the same God as Christians.
Objection 3. It is written: No one who denies the Son has the Father. Whoever confesses the Son has the Father also (1 John 2:23). But Muslims do not believe in Christ’s divinity, and therefore deny the Son. Therefore they do not have the Father and cannot worship Him.
On the contrary, St. Gregory Palamas, writing to his archdiocese while held captive by Muslim Turks, says: Watch not to suffer anything like these ill-minded men; I do not mean in regard to their reverence of God, but rather in regard to their behavior.
I answer that, we cannot say we know something when we do not know its essence, as St. Augustine tells us (On the Trinity, 10.10.16). And he further informs us that we cannot love something that we do not know (10.1.2). Therefore whoever does not know God’s essence cannot love God, and it is clear that whoever does not love God cannot worship Him. But whoever does know God’s essence can, to at least a certain extent, love God and in this way be said to worship Him. By essence we mean that which makes God, God. But God is not God in virtue of being triune, or in virtue of being omnipotent, but by the fact that there is no distinction between His existence and His essence. For this reason, when Moses asks God who He is, God responds by saying: I am The One Who Is (Exodus 3:14, LXX). This is not to say that God being triune, His omnipotence, or His holiness are mere accidents, but rather properties which flow from His essence without being included in it, as St. John Damascene says in the fourth chapter of his exposition on the Orthodox faith: all that we can affirm concerning God does not show forth God’s nature, but only the qualities of His nature. For when you speak of Him as good, and just, and wise, and so forth, you do not tell God’s nature but only the qualities of His nature. Now since Muslims generally acknowledge that God is He Who Is, they therefore grasp His essence and so can be said to know Him and worship Him.
This is also evident from the saints’ teachings. When the Apostle preached in Athens, he said: Athenians, I see how extremely religious you are in every way. For as I went through the city and looked carefully at the objects of your worship, I found among them an altar with the inscription: To An Unknown God. What therefore you worship as unknown, this I proclaim to you (Acts 17:22-23). Like Muslims, the pagans were unaware of the Trinity, yet St. Paul did not say they worshipped a different God, but rather applauded them for worshipping God to the best of their abilities. The Apostle writes elsewhere: For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made (Romans 1:20). Here he clearly states that one may attain knowledge of God without revelation – therefore Muslims can be said to have some knowledge of God and worship Him. This is also clear from what St. John Damascene writes about knowledge of God’s existence and essence in the beginning of his exposition on the Orthodox faith, especially when he says in the eighth chapter that: We believe, then, in One God … believed in and ministered to by all rational creation.
Reply to Objection 1. This has been answered by what has been said above.
Reply to Objection 2. Here Christ is speaking, not of knowing God’s essence, but of knowing God in the most intimate sense.
Reply to Objection 3. Like above, here “has the Father” means “knows God in the most intimate sense.”