For God not to exist, we need to assume -
1. That the universe came into being on its own without divine help
2. That the initial conditions concerning the incredibly fine tuned balance between the rate of expansion of the universe an the force of Gravity (the cosmological constant) which allowed the formation of stars and galaxies was not intentional.
3. That the first living cell came into being through the actions of random, unguided, purposeless events.
4. That a sufficient quantity and quality of beneficial mutations can be generated from random events to drive the process of evolution.
5. That the detrimental effects of harmful mutations could not wipe out life entirely.
6. That every one of the billions of discrete mutations needed to form life as we know it was able to provide sufficient survival advantage in its own right to facilitate natural selection.
7. That molecular activity alone can generate self awareness.
8. That every event needed to write this post was an inevitable consequence of previous events, because we must assume that there can be no such thing as free will.

And we must presume that all these assumptions are true because God does not exist.