The authority of the Church and the Holy Spirit leading the Church. The Church was led by the Holy Spirit in determining which books would be the inspired word of God The Holy Spirit did not then leave the Church but revealed much more in the sacraments and teachings.
Matthew 16:
“Blessed are you, Simon bar Jona, because flesh and blood did not reveal this to you, but My Father who is in heaven. I also say to you that you are Peter, and upon this rock I will build My church; and the gates of Hades will not overpower it. I will give you the keys of the kingdom of heaven;
and whatever you bind on earth shall have been bound in heaven, and whatever you loose on earth shall have been loosed in heaven.”
John 16:12-13
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I have much more to say to you, more than you can now bear. But when He, the Spirit of truth, comes, he will guide you into all truth.
2nd Thessalonians 2:15: "Stand firm Brethren and
safeguard all traditions whether written or word of mouth."
1 Timothy 3:15 I write so that you will know how [l]one ought to conduct himself in the household of God, which is
the Church of the living God, the pillar and support of the truth.
Mark 9: 38-40
John said to Him, “Teacher, we saw someone casting out demons in Your name, and we tried to prevent him because he was not following us.” But Jesus said, “Do not hinder him, for there is no one who will perform a miracle in My name, and be able soon afterward to speak evil of Me.
For he who is not against us is for us.
John 20: 21-23: In his very first Resurrection appearance our Lord to His disciples in the upper room gives this awesome power to his Apostles with the words: “
Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained.” How could they forgive sins if they were not confessed? They could not. This authority comes through the gift of the Holy Spirit which precedes it. You think anyone who says they are Christian has the authority to forgive sins or hold them bound? No, surely not. It is given to the Church and only to those ordained, the apostles and their successors.
From
http://www.catholicfaithandreason.org/confession-of-sins---how-biblical-is-it.html
Nor is the Bible the only witness. We also have the Early Fathers of the Church, who were defenders of the faith in the early Church, often giving their blood in martyrdom for the faith they defended and serving Christ faithfully despite persecution and dangers. We see the evidence of confession of sins in one of the oldest documents from the early Church, the so-called Didache which is the teaching of the Lord to the twelve Apostles, mentioned by Bishop Eusebius, the father of Church history, in Ecclesiastical History, his history of early Christianity. The Didache was divided into three parts, first the "Two Ways" (the Way of Life and Death); second, the rituals dealing with Baptism, Communion and fasting; third, a concluding chapter dealing with Ministry.
"Confess your sins in church, and do not go up to your prayer with an evil conscience. This is the way of life. . . . On the Lord’s Day gather together, break bread, and give thanks, after confessing your transgressions so that your sacrifice may be pure" (Didache 4:14, 14:1 [A.D. 70-90]).