To say that my case for god's existence is ‘defeasible' is to say:

The word defeasible means that something can be challenged, that that particular claim cannot be proven or believed based on evidence. So one can say that God exists, but that argument is defeasible given that they cannot actually prove the existence of God. On the other hand, the argument of a person who claims that God doesn't exist is equally defeasible because there is no proof for that either.