Often the root cause is one faction encroaching on another's land. Sometimes those factions are aligned along religious lines, other times ethnic or other identities.

However religion is almost always used as a tool to further entrench the sides in their willingness(and even zeal) to continue fighting.

I'm absolutely no fan of institutional religion at all. But religion is often used as an excuse to go to war. It's rarely the actual reason, but a tool to polarize groups. Yes, there were a lot of wars seemingly for religious reasons. But if there's no religious reasons than people artificially created them by creating different interpretation inside the same religion. And if that doesn't work, they find other reasons like etnicity, culture or politics. So it's hard to pretend that religion is actually the root cause.

Fascist theocracy causes war, obviously. Religion can be personal, it's when it's politicised the issues arise. See the US, Israel, Iran, India.