The problem is that I failed to distinguish between having justified belief, and being able to justify ones belief. The difference is that we can have justified belief without being aware of it. On externalism, the justification for a belief can be something that the subject might not even have access to, like the causal history that produced his belief. But justifying ones belief is an action rational people perform, a sort of giving of an explanation or an account of how one is justified in holding that belief.
It follows from externalism that, for all I know, the skeptical theist might be justified in believing God always tells the truth—my premise (3) is indefensible. And yet the spirit of my argument persists. If you think about the role divine revelation plays, it's always intended to account for the justification of religious belief. Religious folk would say "God has told us these things and therefore they are true," implicitly assuming that God is not lying.
But now it's clear that the implicit assumption is not plausible so long as we're committed to skeptical theism. After all, for all the skeptical theist knows, God could have a morally sufficient reason to lie about religious matters. (See the previous post for a more in depth analysis of this).
So while the skeptical theists' religious commitments might be justified, they are not something he can justify. And, since we should exercise a healthy skepticism about beliefs we cannot ourselves justify, there is still tension between skeptical theism and religious belief. It seems, at least, the skeptical theist should be no more confident about his religious beliefs than he thinks is appropriate for gratuitous evil.
We can re-formalize this argument as follows, where P is the sort of belief we can only justify by appealing to divine revelation (namely, religious belief):
- Without appealing to divine revelation, there is no way to justify the belief that P
- Skeptical theists cannot appeal to divine revelation to justify their beliefs
- Therefore, skeptical theists cannot justify their belief that P
- Doubt should be reserved for beliefs we cannot justify
- Therefore, skeptical theists should reserve doubt for their belief that P