I recently received the following email from a friend. I thought I would post my reply here. Read it below the fold.
Do you think that Muslims and Christians worship the same God (i.e. is Allah of Qur'an the same God as God of the Bible)? Obviously there are some major differences concerning how God is described in each, but do you think that means we are worshipping different Gods? What aspects of God are essential to believe in so that you are actually worshipping Him?
First off, I like thinking about these issues a lot but I am by no means an expert, especially when it comes to Islam. For my purposes here, I'll assume that substantial differences between Islam and Christianity exist about the nature and divinity of Christ. Christians affirm Christ's divinity and, by extension, the doctrine of the Trinity, while Islam would reject both. If that's not a fair assessment, then take what follows with a grain of salt.
Allow me to reframe your question a bit, just for clarity's sake. I think you are asking something like, "If "Christianity" and "Islam" constitute two different perspectives on "God," are they both looking at the same being?
Presume for a second the answer to that question is "yes." You're in good company--I am under the impression that is the official Catholic position since Vatican II. What this means, though, is that if they are looking at the same being, at the very least they have substantial disagreements about what they are looking at. If they are worshipping God as they perceive Him--and I fail to see how they could not be--then it seems they are worshipping a "different" God. For Christianity, the nature of that God exists in Three Persons. For Islam, it does not. Even if they are looking at the same being, one position is wrong in its conclusions about "Him."
I thin this poses a real problem, though. Suppose for the sake of argument the Christian conception of God is accurate--that "God" means "Three Persons, One Essence" and that our knowledge of that "God" depends upon the special revelation of God-in-Jesus Christ. In other words, it is on the basis of the actual incarnation of Jesus and our subsequent knowledge of it in Scripture that we have formulated our understanding of God. If the Christian conception of God is accurate, then it means Islam is worshipping "not-God." By rejecting God in Jesus Christ, they reject the revelation of the Transcendant Being. The revelation of God in Jesus Christ and through Scripture excludes the possibility of religious pluralism. There is no "GOd" behind the Trinity.
The question of religious pluralism, then, is not a question about salvation. Using arguments (better, questions) such as "Well, what about all the Islamic people? And the infants too, huh?" doesn't address the issue. Religious pluralism is a question of revelation, not salvation. It is a question of God's identity, not His saving grace. Hence, saying that Muslims worship a different God does not necessarily entail they will all go to hell. It simply means that if they go to heaven, it will be (presuming Christianity is true) on the basis of the identity and saving work of Jesus Christ.
Either way, I think there is good reason to be wary of the "creeping imperialism" that many Christians seem to have: "Well, when they worship Allah they are really worshipping God the Father." They're not, in fact, doing any such thing any more than we are really worshipping Allah when we worship Jesus. Why Christians do this is, I think, subject for another email.
Finally, as for "how much is enough to believe to be worshipping the right God," I think I pointed toward an answer above. I am pretty sure we've got to affirm the Trinity in order to get it right--and while our actual understanding of the doctrine may not be perfect, there is no reason to suspend affirmation until it is made so. Faith precedes seeing, as it were.
These are complex issues--my understanding of them is far from complete. I would highly recommend this collection of essays on the topic, especially Kevin VanHoozer's article (chapter three, I believe). The above argument is his, only more convoluted and condensed. I have, not surprisingly, been completely persuaded by it. Either way, it is a great collection of essays.