The Holy Spirit, who the Father will send in My name, will teach you everything. (John 14:26)
These are the words of our Lord Jesus to the Apostles at the Last Supper. He was explaining to them what would happen after He ascended into heaven.
That was the beginning of the Catholic Church.
The Lord Jesus had taught the Apostles many things while He was on the earth. There would be many other things He would teach them from heaven. As He promised, He sent the Holy Spirit to guide His Church.
In the Acts of Apostles, St. Luke recounts the first Church Council. The situation was this: The Apostles had gone out from Jerusalem to preach the Gospel. In the surrounding countries, both Jews and non-Jews came to believe. This left the Apostles with an honest question.