The logic of the matter is that since contingent effects require causes, and contingent causes require causes, there must be a first cause that is not contingent but rather has to exist.
This being the case, if we ask “What if there were no God?” we are trying to draw conclusions from an impossible premise, which is a recipe for nonsensical conclusions.
But of course people do ask the question. It is not a logical but a psychological enterprise.
Very well, let us consider the psychology. The sequence of ideas tends to work out something like this.
If God is dead, everything is permitted.
We’re heard that one.
But if Man is the image of God, Man is dead too.
And then everything is even more permitted.