Mosques were shut and turned into sheds etc. The crackdown on all religions was hard, but Islam was a particular threat to the communists due to its link with nationalism.
What time frame and geography are we talking about? In the 50-s and 60-s the USSR was happy to promote every nationalism except on its own territory, due to its support of colonial uprisings. Was Chombe nationalist and Lumumba internationalist? They'd go with whoever asked them, and put ideological veneer on it.
In International period (20s-30s), yeah, that was the ideological imperative.