It doesn't matter if morality preceded Christianity
For the sake of your argument, it sure as shit does. You are holding Christianity up as this standard of morality when it's not for me, it's not for most people in the world. Your imposition of that standard is what I have a problem with. So if you want to go around, thinking you're moral because you're a Christian, fine. Do that. But if you start going around imposing that religious morality on others, that's when folks get pissed. Christianity
is not the standard. Never has been, never will be. In the history of great religions, Christianity is a blip. It's a patchwork mess of morality tales, patriarchy, and genocide. If you don't think you can be moral without being Christian, then you're a dogmatic fundamentalist and no better than those in Iran.