But isn't it already? I mean isn't St. Peter's the mother church? Sorry to disappoint but it isn't. Well, what about St. John Lateran? No, not even that one, although it is the Pope's cathedral church as Bishop of Rome. No, the first church, the one in which mass was first celebrated, the one in which the apostles received the Holy Spirit and which continued to be in regular use until the fall of Jerusalem was a far more humble structure. It was a room on the upper floor of a synagogue known as the Church of the Apostles or simply the Cenacle.
"Go into the city, and a man carrying a jar of water will meet you; follow him, and wherever he enters, say to the householder, 'The Teacher says, Where is my guest room, where I am to eat the passover with my disciples? And he will show you a large upper room furnished and ready, there prepare for us." Mk 14: 13-15
Then they returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a sabbath day's journey away; and when they had entered, they went up to the upper room where they were staying. Peter and John and James and Andrew, Philip and Thomas, Bartholemew and Matthew, James the son of Alphaeus and Simon the Zealot and Judas the son of James. All these with one accord devoted themselves to prayer together with the women and Mary the mother of Jesus, and with his brothers. Acts 1:13