What is the ruling on someone who enters the mosque without performing prayers and leads the congregational prayer?

One who abandons prayer is in grave danger. It is narrated from the Companions that they deemed him a disbeliever. If he repents and performs the prayers, there is no harm in him leading others in prayer. However, if he leads people in prayer while persisting in abandoning it, this can only be out of mockery or embarrassment. If he then continues to pray regularly, his prayer is valid. But if he reverts to his previous state, it becomes clear that his prayer was not a sincere repentance. As for those among the congregants who were aware of his condition, they must repeat the prayers they performed behind him.