The question is more complicated than you may think
Some time ago I wrote a blog in which I remarked that only Christians worship the true God, that is, the Trinitarian God who is composed of Father, Son and Holy Spirit; God as three divine Persons: creator, saviour and sanctifier. This led to an exchange of emails with a reader of the blog in which we discussed the Muslim idea of God. He thought Muslims worshipped the same God as Christians; I thought they didn’t. He referred me to the philosopher Peter Geach’s book God and the Soul (1969) and specifically to his chapter “On Worshipping the Right God”.
In this chapter, Geach does not specifically refer to the Muslim idea of God but he does make a number of interesting statements. For instance, he writes, “A sufficiently erroneous thought of a God will simply fail to relate to the true and living God at all. Where the line is to be drawn God only knows…” He also comments, “ For Christians, “God” is not a proper name like “Woden”, but a descriptive term, true of the Blessed Trinity and not true of Woden.” In another place he writes, “Only by turning to the true God can a man win grace and forgiveness.”
So do Muslims worship “the true and living God?” According to Fr William Burridge in his chapter “Our Muslim Neighbours (1987) in the CTS Heritage Series entitled Islam, Britain and the Gospel, they do. “Christians and Muslims have many things in common. First, of course, [they have] faith in the same God, the one God who created the world and brings his creatures to their perfection.” Still, Burridge does go on to list the ways in which Islam differs from Christianity, principally the doctrine of the Trinity and the divinity of Christ.
I note that Burridge, writing in 1987, does not mention the importance of missionary work among Muslims. All he says is that “The best witness Christians can give to Muslims of their faith is to be seen by people in whose lives prayer, awareness of God and fidelity to him and his commandments take priority.” This does not seem quite sufficient; after all, Muslims are people of prayer, they are aware of God and faithful to their understanding of him and his commandments. Why bother to try and convert them at all?
Peter Geach is more forthright: after commenting that “We dare not accept a tolerant attitude towards errors concerning the Divine Nature because we are in no position to judge what level of error will entail that a man’s worship is wholly misdirected”, he goes on to conclude that “The right way, surely, is that of the missionary who counts no labour too great to win men from the Kingdom of Darkness.” As I said, Geach doesn’t mention Islam in this chapter, so he will be referring to those people whom we used to call “heathens” or “pagans”, benighted in the full sense of the word.
Nonetheless, although I am neither a philosopher nor theologian and so assume my understanding of these matters is defective, the question still puzzles me: are you worshipping the one true God if you don’t accept the Blessed Trinity? And following on from this, not to know the love of Christ, whose life, death and Resurrection Christians will celebrate at Easter, nor the continuing life-giving activity of the Holy Spirit in the world, is surely a loss greater than we can possibly imagine?