Though it needs to be pointed out the Pope is not God, has never spoken to God, and thus truly has no say in who does or does not get into heaven lol.
To claim you personally know the way an all powerful deity behaves seems the height of arrogance, especially since it's not like every Pope has been all "atheists can totally get in".
We are told it's a sin not to worship God, and since an atheist sure as hell isn't going to be going to confession this means you can sin and not repent and still get into heaven. Which if you can break one commandment and get in, why shouldn't you be able to break others, not repent, and get in? It makes it hard to take "God" and his rules seriously.