[nullc]

How would microbial life on mars be a confirmation of atheists beliefs?

The nature of general religious beliefs is that they're essentially non-falsifiable.

And particularly in this case, life on mars wouldn't be shocking due to the fact that there is life on earth and there is a small amount of mass transfer. If there isn't some amount of microbial life on mars it would only because of the dumb luck that there hasn't happened to be anything on earth that could get transferred, survive the transfer, and survive on mars.