In one sense I would say that if Christianity did not exist then we wouldn't be having this conversation because the universe would cease to exist.
And why exactly would the universe not exist if Christianity didn't?
Help start the United States 
Well, if by that you mean encourage people to come here because of religious prosecution (which sometimes they were guilty of themselves after they got here) and making the founders realize they needed a secular government which doesn't favor one religion over any others, sure. But that's more like what Christianity had done to make people think on what not to do rather than Christianity directly providing something.
On the plus side it can be argued that Christianity has inspired many of man's greatest works of art. Also, many rather famous scientists where inspired to pursue their scientific careers because of their religion.
Of course it could be argued that we'd be further ahead if Christianity never came about, but really we can't say for sure - some other religion very may well have gained popularity if Christianity hadn't been around and it could have been much worse for mankind.
