I've been asked two questions concerning Hitler's relationship with the Catholic Church that seem to demonstrate the Church's culpability in the Holocaust: Why wasn't Hitler excommunicated from the Catholic Church? And, why wasn't Mein Kampf put on the Index of Forbidden Books?
It is true that Mein Kampf was never banned by the Catholic Church; however, this does not mean that the Church approved of the book. Mein Kampf was examined by the Vatican for three years before deciding not to ban the book. They had more than enough reason to ban the book, but they had a good reason not to, as well as something better than simply banning the book.
Although the Church did not approve of Hitler, he did come to power legally; thus, the best thing the Church could do was to get Germany to sign a concordat to secure certian rights of the Church within Germany. Banning a book written by the chancellor of Germany would not have been a wise diplomatic move, and would have likely hindered getting the concordat signed. Getting Germany to sign the concordat, however, gave the Church the right to do something better than banning Mein Kampf. Pius XI's 1937 encyclical, Mit Brennender Sorge (With Burning Concern), was a direct assault on the Nazi ideology and a critique of Mein Kampf. It was not written in Latin, which is the usual language for papal encyclicals, but in German. Without any pre-announcement, copies were smuggled in to Germany and read from the pulpit of every Catholic church in Germany on Palm Sunday (Mass was two hours that day).
Mit Brennender Sorge was the first official denunciation of Nazism made by any major organisation, and because of the concordat that the Nazis had signed with the Catholic Church four years earlier, Catholic priests could legally read it from the pulpit. This did not stop Nazi reprisal, but it did help. The Catholic Church was not able to make such a massive affront to the Nazis again; however, the French did air drop 88,000 copies of Pius XII's 1939 encyclical, Summi Pontificatus (On the Unity of Human Society), over Germany as Ally propaganda.
These two encyclicals make it very clear that the Catholic Church did not approve of Hitler or the Nazis; however, some say the Catholic Church could have taken an even further step in opposition by excommunicating Hitler.
Excommunicating Hitler would have been pointless. He left the Catholic Faith when he left his parents' home. His own description of his religious beliefs was as "a complete pagan." Nonetheless, any Catholic priest could have refused the sacraments to him because he would have been excommunicated ipso facto due to his numerous crimes. There is no evidence that Hitler ever attempted to receive the sacraments after his childhood, and since the only reason for excommunication is to help a sinner recognise the gravity of his sin, thus leading him to seek forgiveness, it would have achieved nothing in Hitler's case.
The Church would have formally excommunicated Hitler if she felt that it would have had some positive effect, and, in the interest of both Christian and Jewish lives, the Church chose not to ban Mein Kampf.