If God existed, where would we get the evidence? How would we get it?
You've omitted part of the question, which should read "If God exists
and interacts/affects the world, where would we get the evidence?"
And that's simple, any such interaction/effect would leave a trace, and that trace would be evidence.
If God exists but does
not interact/effect the world, then it doesn't matter to you, because you are of the world.
As I see it there are only three possibilities:
1. God exists and there is evidence so we should look for the evidence.
2. God exists but there is no evidence so there is nothing to look for.
3. God does not exist and that is why there is no evidence.
Well, those are just my point. And to your point number 1, if you can't find any evidence, then it is certainly possible that God exists -- but is in no way involved with the world. Therefore nothing you do affects God and nothing God does affects you, and you need no longer consider the question at all. Points 2 and 3 speak to that very thing.
I believe (1) God exists and there is evidence, because if there was no evidence God could not hold humans accountable for believing in Him. Why would God expect us to believe He exists and provide no evidence? That would be unfair as well as unreasonable.
Okay then, what is the evidence? You claim that it is in the existence of "Messengers of God." But how do you know who they are, when there have been so very many claimants? Most of the religions of the world exist
because they have each accepted some -- but not other -- such claimants.