Answer:
Patriarch Michael Cerularius's response to being excommunicated by the representative of Pope Leo IX in 1054 was that he excommunicated the representative and pope himself.
Step-by-step explanation:
Humbert visited the Hagia Sophia, Constantinople's church, on July 16, 1054, and excommunicated Cerularius and his clergy. In return, Cerularius summoned a Holy Synod and had all the legates excommunicated.