Atheists: What is 1+1?

Depends.

In a number base > 2, it's 2.

In base 2 it's 10.

In unary (base 1 / tally counting) it's 11.

That is of course, assuming positive integer number bases. You can define non-integer bases, it's just that they aren't particularly useful for counting everyday items. Pi-based number system is useful for counting items in circular geometry, but it's easier to just use an integer base and introduce a multiplicative constant ... pi ... even if that constant ends up being transcendental.

1+1

1+1 ¿

1+1 is a mathematical equation

I am inclined to say 2

Seeing as how the idiot forgot the equal sign, this isn't even an equation so don't answer with 2.

An addition.

One plus one is two.

I know because I can verify it for myself, unlike other things.

A sum

Whatever magic Jesus says it is.