How to easily defeat any argument for God

[Belacqua]

Theology itself begins with the assumption that God exists and contains the assumption that God, in order to be God, must not be like other entities in existence.

When/if theologians try to demonstrate God's existence through logic, they would then be doing philosophy, not theology.

The field of Natural Theology attempts to use reason alone to say that God exists, and what it's like. 
If you want to declare by fiat that Natural Theology isn't really theology, I guess you can. But you'll be disagreeing with every other source, including Wikipedia, which says: 

Natural theology, once also termed physico-theology, is a type of theology that provides arguments for the existence of God based on reason and ordinary experience of .

Anyway, the main point still stands: if you declare that a person's argument must be wrong because his motivations are bad, that's a textbook example of an ad hominem fallacy. Even if we could know what a person's motivations are, the argument still might be right.