Ask them to prove that any of the gods they don't believe in, do not exist.
Tell them that you will use the same method to prove theirs doesn't.
Tell them that you will use the same method to prove theirs doesn't.
You'd believe if you just opened your heart" is a terrible argument for religion. It's basically saying, "If you bias yourself enough, you can convince yourself that this is true." If religion were true, people wouldn't need faith to believe it -- it would be supported by good evidence.