Your thought experiment contradicts your assertion that atheism is “merely lack of belief.” If God literally knocked on an atheists door and said hi, then there would be extremely strong evidence for the existence of God. However, despite this evidence, the atheist would continue to posit that there is no God. It isn’t a lack of belief, but instead a belief that there is no God.
But for an atheist that’s not a good reason to worship. Sure He knocked on the door, so now we know He exists, but so what. Here look at the dictionary definition of belief:
http://dictionary.reference.com/browse/belief
1.
something believed; an opinion or conviction: a belief that the earth is flat.
2.
confidence in the truth or existence of something not immediately susceptible to rigorous proof: a statement unworthy of belief.
3.
confidence; faith; trust: a child’s belief in his parents.
4.
a religious tenet or tenets; religious creed or faith: the Christian belief.
Especially look at #4, most atheists aren’t going to suddenly join a religion because God knocked on the door. We’ve been living our lives just fine not in a religion and having God drop by isn’t a good reason to change that. Because it IS a lack of belief in God for most. It’s a passive verb situation, not active.