With the assumption (belief) that the Messengers are speaking for God, why wouldn't they tell the truth, as God wants us to know it? Granted, they do not reveal everything that God knows, but that is because we don't need to know everything that God knows, and what we do need to know is revealed progressively, in stages, as we need it and are able to understand it.
They might indeed faithfully repeat what they have been told, which is as near is they can get to "truth". But if God doesn't want us to know things then we have no way of knowing what is factual or what has been edited for our consumption. We also have a huge assumption that God is doing things for our benefit. We have no way of verifying that either.
That is a statement based upon logic. If God exists and if God wanted to be verifiable, and if God is omnipotent, God could make Himself verifiable.
Maybe. If God is by its nature beyond our understanding then there might be no way to make us understand without altering our basic nature. Or a hundred and one different explanations that we can consider to be feasible, and all simply guesses, lacking any certainty about the nature of God.
I think we should doubt religious claims unless we have scrutinized them thoroughly and to our satisfaction.
Yes, and very wise, but there's still only so far we can go in the circumstances.
What on earth would God do if humans stopped praying and worshiping Him? God might develop an inferiority complex.
It depends on whether God actually wants us to do that. And once again, we can't know that. Though I'm suggesting this flippantly, I actually feel it might be a good thing for humans to do. It would be the ultimate attempt to make sense of God. If he responded in some way, that would solve so many questions. If he didn't respond (my expected outcome), we could assume that either God wasn't interested in communicating with us, or didn't exist, which are pretty much equivalent, and we could stop wasting time on religion.
(To adherents of various "Eastern" religions, yes I know it doesn't apply to them).