How did religion play a part in the crusades?

Regardless, religion was always the primary trigger for a crusade. Though religion always played a part, the perceived threat to the church evolved. The first Crusade focused solely on ridding the Holy Land of Muslim forces ... The popes began to use crusades to combat any perceived threat against the Catholic Church.

they fought for the right for that country/cultures religion to be spread and to spread over another countries religion to gain population for the churches in Europe.