Originally Posted by Rotaerk

If his mistake was in choosing premises, then don't blame logic, blame his choices. Since knowledge does not exist without premises/axioms, one must choose the premises of ones knowledge. Naturally, if you disagree on premises, your logical deductions, i.e. beliefs, will be different as well. For instance, an empiricist's axioms would include "If I observe the occurrence X, 'I observed the occurrence X' is absolutely true." An axiom that you have chosen is that God exists. Both beliefs are by choice, not necessity, and both lead to different logical deductions.