- Everything that exists has an explanation of its existence (either in its own nature or in an external cause).
- If the universe has an explanation of its existence, that explanation is God.
- The universe exists.
- Therefore, the explanation of the universe is God.
In other words, Craig is arguing that the probability of evidence is affected by the ability of a hypothesis to explain that evidence.
If you have ever been amazed by the way some theists simply dismiss good evidence against their position and unquestioningly accept any evidence in support, no matter how flimsy it may be, this might give you some clue as to why.
I cannot explain this good evidence in light of my hypothesis that God exists, therefore I reject this good evidence. (There are no fossils! Have you looked in a museum? … There are no fossils and no museums!)
In the light of my hypothesis that God exists, this flimsy evidence is probable because even an idiot can explain it in terms of God, therefore I accept this flimsy evidence. (The Earth is perfectly designed for human habitation - so long as we ignore storms, earthquakes, droughts, solar radiation, disease, large infertile regions, depleted fish stocks, etc etc)