After realizing that Memory Alteration can't erase old memories, Saito gave Kein a hint on what they should do. And that is for Kein to allow himself to be injured by Alfred/God Geredes like he did against the Demon King before. Saito would then send Bishop to heal Kein. In the flashback it was mentioned that through Bishop, mana can be transferred from the Saintess to Saito to Kein. In the present since Saito is only half as powerful under Nightmare's influence he doesn't have enough mana to defeat Alfred. So as a final move Kein sent the powers of the Saintess and Kashitsu, which he stole at the beginning of the arc, to Bishop to exorcise Nightmare out of Alfred. Saito then finalize the contract with Nightmare by eating snacks at the same time.