A pretty basic tenet of any religion (or at least the ones that have become dominant today) is that THIS religion is the ONLY ONE TRUE religion, all else being heresy
Most religions make no such claim. The vast majority of religions are "tribal" and make no effort to proselytize to outsiders. Go to your local synagogue and tell the rabbi that you want to convert to Judaism. Most likely he will try to talk you out of it. If you go to a Hindu temple, you will likely encounter similar rejection. Buddhists will be more welcoming, but they make no claim to be the "ONE TRUE" religion, and many don't even consider Buddhism to be a religion. More of a philosophy.
In fact if A and B were in a religious war
There is no such thing as a "religious war". Wars are fought for power and resources. Religion is just a justification, and as a way to motivate poor fools to die for the benefit of rich leaders. The Thirty Years War was by far the worst "religious war" in Europe, yet both Catholic France and Muslim Turkey fought on the "protestant" side.