Why do communists persecute Christians?

At first, the Soviet Communist Party saw Christian churches as institutions capable of rivalling it for the allegience of Soviet citizens. However, during the Second World War, Stalin realised that the Church could be of positive help to the state in mobilising public support for the defence of the Motherland. After the war ended, it became clear that the Church would continue to be of help in furthering the Soviet state's post-war political agenda. Thus, the Soviet Union initially attempted to destroy the Church, then did what it could to assist the Church, on an understanding that the Church would cooperate with the state. Church attendance grew steadily in the post-war period, until the end of communist rule, but has fallen since.