The Catholic church owed allegiance to the Pope and had their own schools so were potentially a threat to Hitler’s control of the population.
Hitler reached a Concordat (agreement) with the Pope in July 1933, in which Hitler promised to allow Catholic religious freedom and schools to operate in return for priests avoiding discussing politics and swearing an oath of allegiance to the State
Hitler did not keep the agreement. Catholic schools were closed or brought in line with Nazi ideas, the Catholic Youth League was banned and any critical Catholic priests were sent to concentration camps.
By 1937, Pope Pius XI realised the Concordat was worthless so issued a critical public statement (Papal Bull) known as ‘With Burning Anxiety’ criticising Nazi policies.