I recently had a conversation with science blogger Gumby the Cat, who said this in the comments:

I do think though, that either all religions (Islam, Judaism, Christianity and the non-Abrahamic religions) were originally inspired by God, or none of them are. I think the tendency of the adherents of one particular faith to ascribe "The One Truth" status to their religion is a man-made conceit. How could one possibly know that?

Is it logical to believe that there is only one true religion?

Let me first make some postulates:

1. God is perfect and unchanging. In another manner of speaking, he is self-consistent and non-contradictory.
2. Many of the world's religions contradict each other. For example, Islam and Christianity have different views of God, both of which are radically different from the Hindu view of God/gods, which in turn is different from Buddhist and Shinto views.

It follows, then, that all religions cannot be true. Therefore at least some of them must be at least partially false.

An atheist might conclude that no religions are true. This does not necessarily follow from my two postulates; they are equally consistent with the view that exactly one religion is completely true. I do not think it is logical to believe that more than one religion is completely true, though.

So either one religion is completely true, or none are. If God is truth, that means that either one religion is "the one" God wants us to follow, or none are. I believe that one is, and I believe this is more logical than believing that many religions are equal in God's eyes.