Reports said new party rules would prevent retired officials from joining any religion and 'cults'.
Christianity has come under repeated attack in China with pastors routinely detained.
A programme to remove crosses from the roofs of churches is ongoing across the communist nation.
China claims to guarantee religious freedom but party members have been expected to remain atheist.
An official is quoted in state media as saying: "There are clear rules that retired cadres and party members cannot believe in religion, cannot take part in religious activities, and must resolutely fight against cults."
"They must maintain a high degree of consistency, in thought, in political views and in action, with the central party committee which is headed by Xi Jinping".
