- 1: God affects the universe in that "The universe exists, scientific laws exist and people have faith in him."
_"The universe exists..."_
First of all, this isn't even really an argument for God. "X exists, therefore God made X" does not follow logically -- any more than "X exists, therefore you made X" or "X exists, therefore my friend George the Invisible 10-foot-tall Pumpkin Pie made X" or "X exists, therefore The Flying Spaghetti Monster made X".
Even if I grant you a pass on that and allow it as an argument, though, it falls apart.
The universe existing argues _against_ the existence of a creator-God, because if everything in existence has to be created by something else, then that "something else" had to have a creator too. There must therefore have been a creator-creator, who must in turn have been created by a creator-creator-creator, and so on ad infinitum.
In other words, if you suppose that the universe was created by a being, you further imply an indefinite chain of creator-beings necessary in order to get to that point. Why has nobody argued for the existence of these beings at all, much less argued with a conviction equal to that of their belief in the existence of God? Why presuppose one creator, but not the rest?
Even if you make a special pleading that _this_ God is so fabulous that the universe needed him in order to be created but he himself didn't need to be created by anything, it still adds a completely unnecessary layer of explanation. Why does the universe have to have been created by a conscious entity, anymore than the sun, the galaxy, or waterfalls or rainbows, needed to be created by a conscious entity?
We know how each of these things were created. All the evidence points to the operation of simple natural laws not subject to control by any outside entity, much less a conscious one. Similarly, we are gradually learning how the universe came to be, and there is no evidence of any conscious interference or design in that creation.
A more likely explanation is that "God did it" is both a _curiosity-stopper_ -- a kind of cognitive divide-by-zero error -- combined with a massive power-grab:
God can do anything, including conceal all the evidence of his own existence, therefore you can answer any question with "God did it" until you figure out the real reason -- and every time you do that, the young children to whom you are teaching this anti-information become more and more awed about how amazingly powerful this nonexistent entity must be.
And since you are now an authoritative source of information on this nonexistent being -- who will never show up in person to contradict your interpretation of his wishes -- you can make any damn claim you want about what God wants, and a large number of gullible souls will believe you -- and in their awe at the stunning awesomeness of this imaginary being you have described, they will do pretty much anything you tell them that God wants them to do.
And that's the answer to your third question, of why people believe in God.
As for your second question -- it's basically the "X exists, therefore God created X" claim again, which I've already answered.
And finally: if you are defining God as "that which causes things to be how they already are", then that God isn't having any detectable effect on the universe -- since things would have gotten that way through the unaided operation of natural laws in the first place.
I suggest you read up on God of the Gaps: http://en.wikipedia.org/wiki/God_of_the_gaps
2 - God of the Gaps
Because science is not absolute, the gaps in which one can credibly insert "God did it" -- because we don't yet have enough evidence to understand what actually happened -- will never completely disappear; there can never be perfect and complete understanding of the universe. What is the value of saying "God did it" about anything, though, when every time we learn how something _actually_ works, there never seem to be any supernatural beings or forces involved?