Poland is a bit of an exception among the Warsaw Pact states precisely because the government was obliged to treat Catholicism with kid gloves.

Any review of the recent history of Poland will find multiple examples of what seems to be an unusually high level of tolerance, and even encouragement, of Catholicism in communist Poland. The elevation of John Paul II to the Papacy was welcomed by the government, and they encouraged him to make an official visit to his homeland. Priests were allowed to say almost anything they wanted from the pulpit, including criticism of the regime. One notable example is Jerzy Popieluszko, an anti-communist priest who was extremely forthright in his criticisms of the government. He was arrested in 1983 but the government released him after other Catholic figures protested. The secret police eventually assassinated him, which may not sound like a sign of indulgence towards the Church, but the officers responsible were themselves arrested and imprisoned by their own government, which refused to condone the murder. There was even an independent Catholic University operating in Lublin, the only University in the whole Eastern Bloc with no party presence, no classes on Marxism-Leninism, etc etc. There were also Catholic newspapers that published independently.

This indulgence wasn't uniform. During the early 1950s, until the change in Poland's leadership in 1956, there were attempts to crack down on the Catholic church, which included closing down some departments of the Lublin university (but even then, not shutting it) and imprisoning Cardinal Wyszynski.

So the answer is that the Polish communist authorities made only a brief, unsuccessful effort to crack down on the Catholic Church. This isn't a real answer, though - the question remains, why did they feel unable to take measures similar to their comrades in the USSR, Czechoslovakia or the DDR? The answer is simply the great strength of Catholicism in Poland. In 1945, the Catholic Church was a major component of Polish national identity, and with a Polish population already highly suspicious of the communist authorities, it was necessary to make concessions. It's likely that the early communist leadership only intended to make these concessions temporarily and to then strip the Church of its power when they were more secure, but after the 1953-1956 attempt failed, they settled down with the Church as a more or less permanent feature of Polish life under communism.

Source: Jerzy Lukowski and Hubert Zawadzki, 'A Concise History of Poland'