As soon as they were written! How do we know this? Well, for one thing, Jesus had commissioned His Apostles to do this very thing, both naturally and supernaturally:
* “But when the Helper comes, whom I will send to you from the Father, the Spirit of truth, who proceeds from the Father, he will bear witness about me. And you also will bear witness, because you have been with me from the beginning.” (John 15:26-27 ESV)
The Apostles would be commissioned for this purpose in two ways. They would testify of what they had seen and experienced, and the Spirit would provide the rest. But how?
* “But the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things and bring to your remembrance all that I have said to you.” (John 14:26)
The Spirit would make up for their inability to understand:
* "I still have many things to say to you, but you cannot bear them now. When the Spirit of truth comes, he will guide you into all the truth, for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come. He will glorify me, for he will take what is mine and declare it to you.” (John 16:12-14)
Jesus subsequently sent them off into the world with the Gospel:
* “Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.” (Matthew 28:19-20)
He expected a lot of them. They had to teach ALL that they had been taught. This could only be accomplished by divine assistance.
The Apostles also came to understand that their teachings were indeed given by the Spirit:
* “Now we have received not the spirit of the world, but the Spirit who is from God, that we might understand the things freely given us by God. And we impart this in words not taught by human wisdom but taught by the Spirit, interpreting spiritual truths to those who are spiritual.” (1 Corinthians 2:12-13)
The fact that the Apostles had been divinely commissioned by the Lord to bring the Gospel to the world was made plain to all by Him:
* “How shall we escape if we neglect such a great salvation? It was declared at first by the Lord, and it was attested to us by those who heard, while God also bore witness by signs and wonders and various miracles and by gifts of the Holy Spirit distributed according to his will.” (Hebrews 2:3-4)
These signs and wonders accompanied the Apostles in order to validate their divine commission before the Church, and the Church got the message:
* “And they devoted themselves to the apostles’ teaching and the fellowship, to the breaking of bread and the prayers. And awe came upon every soul, and many wonders and signs were being done through the apostles.” (Acts 2:42-43)
It therefore became clear to all that the Apostles were teaching with the authority of God Himself. It was also these signs that had enabled Paul to declare that he too was speaking and writing the very words of God:
* “The signs of a true apostle were performed among you with utmost patience, with signs and wonders and mighty works.” (2 Corinthians 12:12)
The miracles Paul was performing were an unmistakable divine validation - a sign that God approved of Paul's teaching:
* “So they remained for a long time, speaking boldly for the Lord, who bore witness to the word of his grace, granting signs and wonders to be done by their hands.” (Acts 14:3 ESV)
* “And God was doing extraordinary miracles by the hands of Paul, so that even handkerchiefs or aprons that had touched his skin were carried away to the sick, and their diseases left them and the evil spirits came out of them.” (Acts 19:11-12 ESV)
Consequently, there was never any doubt in the Church that Paul's 13 letters were each the Word of God. And they were received accordingly.
Some scholars claim that the Apostles would never have believed that they were writing Scripture. However, this claim does not accord with the Scriptural evidence. Clearly, Paul knew that he was penning the Word of God:
* “Yet among the mature we do impart wisdom, although it is not a wisdom of this age or of the rulers of this age, who are doomed to pass away. But we impart a secret and hidden wisdom of God, which God decreed before the ages for our glory.” (1 Corinthians 2:6-7)
* “If anyone thinks that he is a prophet, or spiritual, he should acknowledge that the things I am writing to you are a command of the Lord.” (1 Corinthians 14:37)
Not only did Paul declare his writings to be Scripture, he also claimed that this Word could supernaturally transform:
* “And we also thank God constantly for this, that when you received the word of God, which you heard from us, you accepted it not as the word of men but as what it really is, the word of God, which is at work in you believers.” (1 Thessalonians 2:13)
Consequently, his letters were received as Scripture and were copied and carried around to other churches (1 Thess. 5:27; Col. 4:16). Evidently, the various churches regarded his writings as Scripture as soon as they were received.
Peter also regarded his writings as the commandments of the Lord:
* “You should remember the predictions of the holy prophets and the commandment of the Lord and Savior through your apostles,” (2 Peter 3:2)
Peter also regarded Paul's writings as Scripture (2 Peter 3:15-16). And John regarded his as Scripture:
* “I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues described in this book, and if anyone takes away from the words of the book of this prophecy, God will take away his share in the tree of life and in the holy city, which are described in this book.” (Revelation 22:18-19)
The churches didn't have to wait for a Church Council for them to adopt the Apostolic writings as Scripture. No expert pronunciation was necessary. Instead, the Lord had made these decisions quite obvious.
Clearly, only the Apostles had such authority. Consequently, there was only one way to partake of such influence - to pose as an Apostle, attaching an Apostle's name to their own letter.
Therefore, Paul had to warn the Church against such forgeries (2 Thess. 2:2; 3:17; 1 Cor. 16:21; Gal. 6:11; Col. 4:18). Such a warning would only be appropriate if his own letters carried significant weight.
Consequently, it wasn't until about 200 after the Cross that the Church began to seek further assurances for seven Epistles. The other 20 books of the NT were never questioned.