I don't take people claiming that my beliefs are 'temporary conditions' very kindly.
Logically speaking, if an atheist were correct, his or her belief would be a temporary condition, just like the rest of the human experience. If an atheist were incorrect, their beliefs would still have a likelihood of being a temporary condition. If you are uncomfortable with this, it means you are uncomfortable with your own worldview. Don't worry, you don't have to be comfortable with how you see the world--God knows, if my worldview is correct, then it's a very ugly world we live in--but self awareness is the key to outflanking your enemies.
That said, his signature does sound like a threat.