Religious wars were based on political and economic interests to strengthen kingdoms. It started in the 11th century where Muslims are also expanding their territories in Asia Minor and Europe. Catholics, on the other hand, wanted to show their power. Both religions associated their conquest as a duty to God. Winning wars were all about having a stronghold in the land which gave way to territorial expansions.
After this, the Reformation also came out which questioned the religious authority of the Catholics due to corruption and abuse of clerical practices.