Seeing isn't always believing. A magician can deceive people into seeing the impossible. Nor is seeing essential for believing because we obviously believe...
in much that we have never seen (wind, gravity, truth, justice, etc.). If God showed himself, many would probably dismiss it as a hallucination because...
they already hold a philosophy which excludes God in the first place. Does this seem reasonable to you?
If the person asks where's the proof for God/Jesus/Christianity, go here.