What is the ruling on a congregant praying behind an infidel imam, knowing of his infidelity, and does he become an infidel by his action, or is his prayer merely invalidated?

It is not permissible to follow a disbeliever in prayer, and whoever prays behind him—especially with knowledge of his disbelief—his prayer is invalid, because the prayer of a disbeliever is not valid for himself, so it is not valid for others. Some jurists have held the prayer to be valid if the worshiper is unaware of the imam's disbelief. As for the disbelief of one who follows a disbeliever in prayer while knowing of his disbelief, if he did not do so out of mockery, then he is not judged to be a disbeliever, because there is no statement from the jurists on this matter, and declaring someone a disbeliever is a serious matter. The default state of a Muslim is Islam, and he is not expelled from it except with certainty.