There are always better explanations for unexplained phenomena than god: fraud and faulty sensory perception cover most of the bases, but mostly, if I see a Madonna appear in a field to bless me, the first thing I'd suspect is brain damage. We have clumsy, sputtering, inefficient brains that are better designed for spotting rutabagas and triggering rutting behavior at the sight of a curvy buttock than they are for doing math or interpreting the abstract nature of the universe. It is a struggle to be rational and objective, and failures are not evidence for an alternative reality. Heck, we can be fooled rather easily by mere stage magicians; we don't need to invent something as elaborate as a god to explain apparent anomalies.
That last point does imply, though, that there is one path that could convince me of the existence of god: major brain damage. I don't think that wacking me in the skull with a ball-peen hammer counts as evidence, however.