Re "The multiverse is a theory . . . I don't see why atheists would use this theory to support their idea that there is no God.":
The multiverse theory - the idea that there are a possibly infinite number of universes - is uncannily like the idea of the medieval scholastics that in God there were an infinite number of potential worlds that He(?) could actualize. The big difference - and it is a really, really big difference - is that in the multiverse theory all possible universes *must* exist - including the most hideous. For the theologians, God would *only* bring into existence those worlds he felt were good (good in the long run - and for the majority of us). And God saw every thing that he had made, and, behold, it was very good. (Genesis 1:31) Also in this He shewed me a little thing, the quantity of an hazel-nut, in the palm of my hand; and it was as round as a ball. I looked thereupon with eye of my understanding, and thought: What may this be? And it was answered generally thus: It is all that is made. I marvelled how it might last, for methought it might suddenly have fallen to naught for little[ness]. And I was answered in my understanding: It lasteth, and ever shall [last] for that God loveth it. And so All-thing hath the Being by the love of God. (Julian of Norwich) There is no evidence for there being any multiverses - it is as much a leap of faith as believing in God. The "God option" really means you have trust in whatever is unfolding.