I believe that Jesus and the apostles knew they were completing the Biblical cannon. In the upper room discourse Jesus told the apostles that when the Holy Spirit came to replace him as the new parcelete that He would lead them "into ALL truth." The manner in which he would do this is that he would bring to their remembrance those things which he had taught:
Jn. 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
What the Holy Spirit will do is "testify" of Christ and thus produce through them the "testimony" of Christ (Isa. 8:16 "the testimony"):
26 ¶ But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:
27 And ye also shall bear witness, because ye have been with me from the beginning.
The apostle John who penned these words by the direction of the Holy Spirit went on later to say that the "spirit of prophecy is the testimony of Jesus" - Rev. 19:10 and that his final apostolic writing was such a "prophecy" of "the testimony" of Jesus (Rev. 1:2) which should be "READ" in the churches (Rev. 1:3).
Rev. 19:10 And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy.
Rev. 1:2 Who bare record of the word of God, and of the testimony of Jesus Christ, and of all things that he saw.
3 ¶ Blessed is he that readeth, and they that hear the words of this prophecy, and keep those things which are written therein: for the time is at hand.
Moreover, those things of Christ which the Holy Spirit shall shew them include not only what Christ said in his personal ministry but what he has to say about the future (and we find that the final book of the New Testament is completely dedicated to future things:
Jn. 16:13 Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.
14 He shall glorify me: for he shall receive of mine, and shall shew it unto you.
15 All things that the Father hath are mine: therefore said I, that he shall take of mine, and shall shew it unto you.
The New Testament canon is closed by the last living apostle concerning this very subject of things to come shown to him by the Spirit of Christ. Revelation 1:2 have a strong resemblance to Isaiah 8:16 by the words "The testimony and the Law" compared to "the word of God and the testimony." After the law and the testimony are bound and sealed together as "this word" (Isa. 8:20) the next revelation Isaiah expects is the Lord's coming from heaven. Moreover, after John, the last living apostle and disciple who walked with Jesus seals this prophecy (Rev. 22:18-19) the next revelation John expects is the Lord's coming from heaven (Rev. 22:20-21).
However, back to the upper room prediction that his disciples would be led into "all truth" through the Spirit bringing to remembrance what he had said to them, thus producing a testimony of Jesus, then Jesus claims that this testimony of Christ through them would be preserved unto future generations which shall come to faith "through their word."
Jn. 17:17 ¶ Sanctify them through thy truth: thy word is truth.
18 As thou hast sent me into the world, even so have I also sent them into the world.
19 And for their sakes I sanctify myself, that they also might be sanctified through the truth.
20 ¶ Neither pray I for these alone, but for them also which shall believe on me through their word;
He did not say through their "words" (plural) but through the singular "word" as one unified source. This ministry of the Holy Spirit through "my disciples" (Isa. 8:16) to produce "the testimony" of Jesus is what the writer of Hebrews refers to when he says:
Heb. 1:1 ¶ God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, 2 Hath in these last days spoken unto us by his Son,
The Son's words are provided for us and preserved for us through "the word" (Jn. 17:20) of his disciples as the "testimony of Jesus." They are first orally transmitted to the churches (Acts 2:41; 14:4-5) and then perserved in written form for the churches and for future generations. The transition from oral to written form are both equally called "the traditions" recognized by the apostles and New Testament churches as "scriptures" or the Word of God:
2 Thes. 2:15 Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
For our Catholic friends, note the term "traditions" equally apply to both the oral and written record and especially note that the command to keep the oral tradition is contemporary with the churches actually receiving it from the prophetic source within the actual time frame of the apostles who can still authenticate their oral teaching rather than in some future generations. It is the written form that preserves it for future generations which they realized they were producing and affirmed what they were writing was to be the final authority:
2Th 3:14 And if any man obey not our word by this epistle, note that man, and have no company with him, that he may be ashamed.
2 Pet. 1:16 ¶ For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty.....
19 ¶ We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts:
2 Pet. 3:15 And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you;
16 As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.
2 Tim. 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
17 That the man of God may be perfect, throughly furnished unto all good works.
The apostles understood they were producing the testimony of Jesus in written form as scripture. They were fulfilling the prophecy of Isaiah that the law and the testimony would be bound and sealed "among my disciples" and the last living apostle realized his final writing was the completion of "the word of God and the testimony" (Rev. 1:2) that completed the Biblical canon by the revelation of future things from his own time concluded by a new heaven and earth thus sealed "the word of God and the testimony expecting the next revelation from heaven to be the personal visible coming of Jesus Christ:
Rev. 22:18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
20 ¶ He which testifieth these things saith, Surely I come quickly. Amen. Even so, come, Lord Jesus.
Isa. 8:16 ¶ Bind up the testimony, seal the law among my disciples.
17 And I will wait upon the LORD, that hideth his face from the house of Jacob, and I will look for him.
18 Behold, I and the children whom the LORD hath given me are for signs and for wonders in Israel from the LORD of hosts, which dwelleth in mount Zion.
19 And when they shall say unto you, Seek unto them that have familiar spirits, and unto wizards that peep, and that mutter: should not a people seek unto their God? for the living to the dead?
20 To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.