Hint: It is NOT St. Peter's Basilica.
Even though St. Peter’s Basilica in Vatican City is now the primary church where the pope celebrates Mass, it is not the pope’s cathedral, nor is it the “mother church.”
Both of those honors belongs to St. John Lateran in the city of Rome, located outside the confines of Vatican City.
The land where the basilica now stands originally belonged to the Laterani family, but was eventually acquired by the wife of Emperor Constantine. It is said that he gave the land to the pope for his personal residence.
Pope Sylvester consecrated the church on November 9, 324, and it has played a vital role in the life of the Catholic Church for many centuries. According to the Catholic Encyclopedia, “It was dedicated to the Saviour, ‘Basilica Salvatoris,’ the dedication to St. John being of later date, and due to a Benedictine monastery of St. John the Baptist and St. John the Evangelist which adjoined the basilica and where members were charged at one period with the duty of maintaining the services in the church. This later dedication to St. John has now in popular usage altogether superseded the original one.”
Since the pope lived there, the church naturally became his cathedral, the place where his “cathedra” (bishop’s chair) was located. From this chair the pope would govern the Church and it remains the church where the pope, bishop of Rome, offers important Masses.
Where does the pope live?
This long association with the pope and its early dedication is why it is regarded as the “mother church” for all other churches. According to Zenit, “There is a formidable and significant stone inscription on the façade of the Basilica that reads: Sacrosancta Lateranensis ecclesia omnium urbis et orbis ecclesiarum mater et caput, ‘Most Holy Lateran Church, of all the churches in the city and the world, the mother and head.‘”
However, in the past few centuries the pope moved his residence next to St. Peter’s Basilica, the location of the first pope’s tomb. St. Peter’s also became a more practical church because of its ability to hold many more thousands of people than the Lateran basilica.
Despite its decreased significance in recent years, St. John Lateran still holds a special place in the Catholic Church and is revered for its ancient history and close connection to the pope.