The apostles continued to refer to each others work. One of last books written, save the writings of John, was Jude. The author refers to himself as a second generation writer.
Hebrews 2:3-4 How shall we escape, if we neglect so great salvation; which at the first began to be spoken by the Lord, and was confirmed unto us by them that heard him; God also bearing them witness, both with signs and wonders, and with divers miracles, and gifts of the Holy Ghost, according to his own will?
The salvation preached by Christ was confirmed to the writer by the Apostles that heard Christ. God bore the Apostles witness with signs and wonders and miracles and the gifts of the Holy Spirit.
This writer came after the Apostles, not necessarily after they were all dead, but after most of them had already written. He had knowledge of their writings, their authority, their witness to Christ, etc.
Peter says:
2 Peter 3:1-2 This second epistle, beloved, I now write unto you; in both which I stir up your pure minds by way of remembrance: That ye may be mindful of the words which were spoken before by the holy prophets, and of the commandment of us the apostles of the Lord and Saviour:
--Remember this epistle. Be mindful of the words spoken to you: first by the prophets (OT), and secondly of us, the Apostles of the Lord. He puts the words, the writings of the Apostles on the same level as the OT, telling them that they are inspired writings. Peter knew which were inspired and was telling them that the words of the Apostles were inspired.
Jude 1:3 Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints.
--The faith is that body of doctrine which is written down. Jude also was written about 70 A.D., later than all of Paul's epistles, three of the gospels, and the book of Acts, the epistles of Peter and James.
Contend for the faith. What faith? The faith that we have in the NT. It was mostly written by that time. The apostles traveled. Scribes made copies.
2 Timothy 4:13 The cloke that I left at Troas with Carpus, when thou comest, bring with thee, and the books, but especially the parchments.
--What did Paul want? Books, but especially the parchments.
From Rome he wrote many of his epistles.
Jamieson, Faucett and Brown say:
The consensus for the date of the book, "The Shepherd of Hermas" is 140-155 A.D., a date which puts it out of the running to be included in the canon of Scripture. The canon was completed by the end of the first century. The books were all written by Apostles or associates of Apostles. This book has no apostolic authority. Neither do the apocryphal books except in the eyes of the RCC. The OT canon was complete between 400-450 B.C. No Jew accepted any writing after 400 B.C. All the apocryphal books were written after 250 B.C., and some of them after Christ was born. And these are supposed to be in the OT??? Pure fraud!! It demonstrates that they could not have been included in the original Septuagint written in 250 B.C. Thus there is no question that the apocryphal books were never a part of the canon of Scripture.
Sola Scriptura has to do with study, not with canon.
I quoted from Jamieson, Faucett and Brown above. I used him as an authority. However the Bible is my final authority in all matters of faith and practice. That is sola scriptura.
Easy enough. The RCC is wrong, always has been.