This is a tricky question to answer. Officially, China’s CCP is atheist, and any member of the CCP has to be an atheist as well. However, there are still Muslims, Buddhists, and Christians, among others, in China. This brings up an interesting power dynamic. Anyone who wants to worship and not be persecuted for it has to join one of the approved religions and go to a Chinese-government approved place of worship.
This has led to many “house” versions of the religions - where the practitioners keep it secret. A good example of this is the current persecution of Falun Gong (which is more a meditation practice than a religion, but it fits the pattern so well), which had been incredibly popular in China in the 1980s.