It happened when Reiner admitted he was a titan armor and he said that his friend (berlthod) was a colossal titan. He admitted that they were the ones who destroyed Wall Maria and he intended to invite Eren. At first Eren didn't believe it and Armin and several other people were suspicious but they remained silent. After a while there was a fight between Eren and Reiner and Eren lost the fight, he and Ymir were kidnapped by Reiner and berlthod.