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.

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