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.
While He was on earth, the Lord Jesus had never explained whether or not Christians had to follow all the Jewish laws. According to the Old Testament, all men had to be circumcised.
The Apostles had learned the laws of Moses as children. Do these laws apply to all the members of the Christian Church, even those of non-Jewish origin? This was the honest question the Apostles had.
The Apostles gathered back together again in Jerusalem. They prayed and debated to find the answer to their question. As the Lord Jesus had promised, the Holy Spirit helped them to discern the answer.
The Council then composed a letter to be read to the Gentile Christians in Syria.
The letter contains a very interesting and important sentence. It is a sentence we Catholics should keep in our pockets, so to speak. It will help us to explain ourselves to our critics, be they Mormon, Jehovah’s Witness, Evangelical, or any Protestant.
The letter of the Council of Jerusalem was addressed to confused Christians. The first non-Jewish Christians had heard that they were supposed to keep the Law of Moses. The uncircumcised men, who feared God but also feared pain and personal bloodshed, were not sure that they wanted to follow the Law of Moses. They wanted to be baptized into Christ, but they were not looking for minor surgery in a particularly sensitive area.
As it turned out, these men had nothing to fear. It is not the will of God that adult men who seek Baptism also have to be circumcised. It is not necessary. The Lord Jesus shed all the blood that needed to be shed.
But the poor Gentile converts were confused because they had been told otherwise. The letter from the Apostles sympathized with their confusion:
Some of our number went out without any mandate from us and upset you with their teachings and disturbed your peace of mind.
This is the crucial sentence. The Apostles condemned those who taught their own doctrines “without any mandate from us.”
To teach the true religion, the religion of the Lord Jesus Christ himself–to teach God’s religion–a man must have a mandate. No one can give himself a mandate to teach the truth of God. The mandate must come from a higher authority.
Now, non-Catholics might say “Look here, popish brother. My mandate is this here Holy Bible.”
Fair enough. We too love the Bible.
We reply, “Dear Brother, if you please: Open the Bible, and read it. Read the end of the four Holy Gospels and the beginning of the Acts of the Apostles. You will, dear friend, discover that the Lord Jesus never handed anyone a Bible. He never handed anybody a book and said, “This is your mandate, Preacher Man. Set up shop for yourself.” He never did that.
Christ chose His Apostles, consecrated them at the Last Supper, gave them the sacraments, and sent them on their mission. As the Apostles were performing their sacred mission, they wrote the New Testament. The Church came first, the written word second.
The New Testament is our unfailing guide to the true doctrine which originated with Christ Himself. But without the Catholic Church, without the successors of the Apostles, the Pope and bishops—without the living mission of the Church which started in Jerusalem almost two thousand years ago—without that, the Bible is a dead letter. The New Testament is not an independent book. It is a family heirloom of our Church.
We human beings need a teacher with a mandate from God. We can discover and teach many wonderful things on our own. But we cannot know the mysterious truths of Almighty God unless He Himself teaches us. We cannot go up to Heaven, take classes, and come back home to start our own school of divine teaching. We need God to teach us about Himself. We need teachers with more than just a human mandate.
Father Mark White Blog 
That is a great explanation.