How do you know they're not right?
If someone asks 2 people why they think God exists and they both say "I know God exists because I have a personal relationship with God and God transformed me", but each belongs to a different religion (1 Christianity and 1 Hinduism) with different theologies and philosophies, is 'knowing God transformed me' a reliable method? If 2 people use the same method, and come to different conclusions, is their method reliable?