Some Catholic priests opposed Hitler. In 1937, the Pope issued a message called 'With Burning Concern' which was read in every Catholic Church. It described Hitler as “a mad prophet with repulsive arrogance”.
The Catholic Archbishop of Munster led a successful campaign to end euthanasia of mentally-disabled people.
Many Protestant pastors, led by Martin Niemöller, formed the Confessional Church in opposition to Hitler's Reich Church. Niemöller was held in a concentration camp during the period 1937-1945. Another Protestant pastor, Dietrich Bonhöffer, was linked to an assassination attempt against Hitler and was executed in 1945.
The video below describes how the Christian opposition to Hitler failed.