We will attack this in two stages. Firstly, the Cathedral and its history and then the Gospel reading.
Lateran refers to the Roman family who owned the Lateran palace. For some reason it was confiscated by the Roman Emperor Constantine. Constantine converted to Christianity in the fourth century and liberated the Christians. At the time the Pope, Sylvester, was in prison. With the emancipation of the Christians, Pope Sylvester went from prison to a palace, the gift from the Emperor. This became the residence of the Pope 10 centuries before St Peter’s. The Church of the Bishop of Rome is called the “Mother and Head of all the churches”. It is dedicated to John the Baptist.
So the Catholic Church remembers the consecration of this Cathedral every year. We remember the consecration of all our churches throughout the world. We ask God’s blessing again on all our places of worship.
The Gospel is from John and it is the passage of the cleansing of the temple. In the synoptics (Mt, Mk, and Lk) Jesus is hardly ever in Jerusalem. He is there in the infancy narratives, when Jesus was 12 and on the Cross. In the Gospel of John, Jesus is hardly anywhere else! This is Chapter Two. The event is the cleansing of the Temple.
We find people selling cattle and sheep and pigeons. The Law demanded that people offer as part of their tithe the first born of their new stock. You can imagine bringing your very best little lamb 70 miles from say Nazareth to the Temple. The lamb would be weary and in poor condition. The sellers were there to offer you a fresh beautiful lamb for sacrifice at a high price. Jesus whipped them out of the Temple. (The Fathers of the Church took delight in the reference to the pigeon sellers. They sold to the poor and were treated lighter!)
Zeal for your House will devour me. “What sign can you show us to justify what you have done?”
Jesus refers to His death and Resurrection. There is often a reference to seeking a sign in the Gospel.
A summary of Jesus’ teaching is: If you are doing the will of God you do not need a sign. In this case you don’t need a sign to tell you to stop abusing religion. If you are discerning an extraordinary mission, you do need a sign. Jesus’ sign is to foretell the Passion.