Why did Hitler hate Catholics?
Nazi ideology could not accept an autonomous establishment whose legitimacy did not spring from the government. It desired the subordination of the Church to the state.
During the German Occupation of Poland (1939–1945), the Nazis brutally suppressed the Catholic Church in Poland, most severely in German-occupied areas of Poland. Thousands of churches and monasteries were systematically closed, seized or destroyed. As a result, many works of religious art and objects were permanently lost.
Church leaders were especially targeted as part of an overall effort to destroy Polish culture. At least 1,811 members of the Polish clergy were murdered in Nazi concentration camps. An estimated 3,000 members of the clergy were killed. Hitler's plans for the Germanization of the East did not allow Catholicism.
The actions taken against Polish Catholicism were part of Generalplan Ost which, if carried out, would have eventually eradicated the existence of the Poles. Adolf Hitler said in August 1939 that he wanted his Death's Head forces "to kill without pity or mercy all men, women, and children of [ethnic] Polish descent or language."
-- From wikipedia