But as the atheists point out, [the arising of a universe that could sustain life] could happen without God because (assuming the gravity field exists) there is a chance, however small.
Not just “there is a [small] chance.”
I often use this analogy: in a game of bridge, the odds of being dealt a perfect hand (two through ace, all of the same suit) are astronomical – one in several billion, if I remember correctly.
But what are the odds of being dealt a hand like the king of diamonds, the four of clubs, the eight of diamonds, the jack of hearts, etc. (some other hand that we haven’t designated as “special”)?
The odds are exactly the same.
The only difference is that when the “perfect hand” comes out, we already have developed a standard by which to call it “special” – but it’s not, in fact, any rarer than any other combination of cards.
Something similar is going on with this whole “anthropic principle” nonsense. If the universe was “randomly generated” (with a random configuration of variables), then the odds of any particular set of variables arising are exactly the same as any other particular set of variables.
The universe we got is capable of sustaining life, at least on one planet that we know about. If the universe were a little different, perhaps it would have been capable of supporting a different kind of life, or perhaps no life at all would have emerged in a different universe. But in terms of odds, none of those potential universes is any “rarer” than another.
[And, naturally, we’re ignoring the possibility that some cosmic law governs the arising of universes and regulates potential sets of variables]
The point is that you can’t take the result that we got, treat it as “special,” and use it as the standard to measure the “odds,” as if the universe we got were “intended” by something all along – to do that is to presume that there is a god already, which makes your argument circular.