The mentors of Communism thought to replace the Christian doctrine with the communist one. That's why they saw in religion their ideological enemy. For those who lived under the soviet influence it was clear that the communist party, as the single party, and the Inquisition of 14-15 centuries are more similar than different. The communists had one more reasons to be afraid by the church. They started from the idea that, in any democracy, the politicians come to power thanks to the people's vote.
Most people are non educated and easy to handle with simple and lying arguments.
Intellectuals and priests used to be the communists' enemies. That's why the first thing which the Russians did in Romania after the II WW was to kill them. I assure you of their efficiency.