[This was prepared to answer Nihil Sine Deo’s question here https://hermeneutics.stackexchange.com/questions/98632/which-are-all-the-scriptures-paul-had-in-mind. But that is closed]
Answer
Scripture, in 2 Timothy chapter 3, refers to both Old Testament and New Testament writings.
Explanation
Moses knew very well that what he got as revelation from God was only partial. Hence he wrote:
“The secret things belong to the LORD our God, but the things that
are revealed belong to us and to our children forever, that we
may do all the words of this law” (Deut 29:29).
Moses knew that some of God’s secrets were to be revealed later. Hence he said:
“The LORD your God will raise up for you a prophet like me from among
you, from your brothers—it is to him you shall listen” (Deut
18:15).
Apostle John agrees and says:
“For the law was given through Moses (OT); grace and truth came
through Jesus Christ (NT)” (John 1:17).
Moses, as a member of the Levi tribe, was a Jew
who said, “to us and to our children”.
So, what? Apostle Paul says:
“the Jews
were entrusted with the oracles of God (both OT and NT)”
(Rom 3:2).
Jesus Christ agrees:
“for salvation is from the Jews
” (John 4:22).
Old Testament as Scripture
The Old Testament is broadly divided into 3 parts:
Torah = Law;
Nebiyim = the Prophets; and
Khitubim = Writings.
This is known as “TaNaKh”; an acronym of the first three letters.
[In the “Writings” section, Psalms is the first book to appear in order. So sometimes “Writings” is also called as “Psalms”]
Jesus Himself acknowledges this tripartite division of the OT:
“And He said to them, These are the Words which I spoke to you yet
being with you, that all the things must be fulfilled having been
written in the (1) Law of Moses, and the (2) Prophets, and the
(3) Psalms, concerning Me” (Luke 24:44).
Flavius Josephus, an almost contemporary of Jesus Christ has acknowledged the tripartite division of the OT in 22 books (or scrolls):
“We have but twenty-two [books] containing the history of all time,
books that are justly believed in; and of these, five are the books of
Moses, which comprise the law and earliest traditions from the
creation of mankind down to his death. From the death of Moses to the
reign of Artaxerxes, King of Persia, the successor of Xerxes, the
prophets who succeeded Moses wrote the history of the events that
occurred in their own time, in thirteen books. The remaining four
documents comprise hymns to God and practical precepts to men (William
Whiston, trans., Flavius Josephus against Apion, Vol. I, in Josephus,
Complete Works, Grand Rapids: Kregel, 1960, p. 8)
[https://blogs.blueletterbible.org/blb/2012/05/29/josephus-historical-evidence-of-the-old-testament-canon/]
Prophets Knew of More Revelation to Come
Just as Moses knew that another Prophet was to come and reveal more of the secrets of God, Isaiah also knew this and wrote about it:
“Bind up the Testimony, seal the Law among My disciples” (Isa 8:16).
What is the Testimony?:-
A similar verse is found in Revelation which defines the Law and the Testimony:
“And the dragon was enraged over the woman, and went away to make war
with the rest of her seed, those keeping the commandments of God
(OT), and having the testimony of Jesus Christ (NT)” (Rev 12:17).
_Who are “My disciples”?:-
First of all, it is Yahweh who says “My disciples”. So, who are His disciples?
Very surprisingly, the disciples are the Apostles of Jesus Christ!!
Evidence?
The Scripture was to be bound up and sealed (completed) by the disciples of Yahweh.
Who was this Yahweh? Let us see.
“Sanctify Jehovah of Hosts Himself, and let Him be your fear; and
let Him be your dread. And He shall be for a sanctuary, and for a
stone of stumbling, and for a rock of falling to the two houses of Israel; for a trap and for a snare to the ones living in
Jerusalem. And many among them shall stumble and fall, and be
broken, and be snared, and be taken. Bind up the Testimony, seal
the Law among My disciples” (Isaiah 8:13-16).
Both Peter and Paul quoting Isaiah’s prophecy, identify who this Yahweh of Hosts is:
“But to disobeying ones, He (Jesus) is the "Stone which those
building rejected; this One became the Head of the Corner, and a
Stone-of-stumbling, and a Rock-of-offense" to the ones stumbling, being disobedient to the Word, to which they were also
appointed” (1 Pet 2:7-8).
“For they stumbled at the Stone-of-stumbling, as it has been
written, "Behold, I place in" "Zion a Stone-of-stumbling," "and a
Rock-of-offense," "and everyone believing on Him will not be shamed” (Rom 9:32-33).
Yes, both Peter and Paul identified the Yahweh of Hosts as Jesus Christ. This is undeniable!
New Testament as Scripture
So, it was Jesus’ disciples who were predestinated to complete and bind up the Scripture. Thus, bind up and seal the Scripture (both OT/Law and NT/Testimony of Jesus) was to be done by the disciples of Jesus.
Peter and Paul Knew This
Both Peter and Paul knew that they were to complete the Scripture, the Oracles of God. So, the NT part of the Oracles of God was also written by Jews, as written in Romans 3:2.
Peter identifies Paul’s epistles as Scripture:
“as also our beloved brother Paul wrote to you, according to the
wisdom given to him; as also in all his epistles, speaking in them
concerning these things, in which are some things hard to understand,
which the unlearned and unsettled pervert, as also they do the rest
of the Scriptures, to their own destruction” (2 Pet 3:16).
“Rest of the Scripture” means clearly that Peter considers Paul’s writings as Scripture.
Paul identifies Luke’s gospel as Scripture:
“For the Scripture says, "You shall not muzzle an ox treading out
grain," and, “the laborer is worthy of his pay
” (1 Tim 5:18).
Here Paul considers Deuteronomy 25:4 and Luke 10:7 (laborer’s pay)
on an equal footing.
This clearly shows that Paul considered Luke as Scripture!
Peter Arranges New Testament
“I think it only right for me to stir up your memory of these matters
as long as I am still alive. I know that I shall soon put off this
mortal body, as our Lord Jesus Christ plainly told me. I will do my
best, then, to provide a way for you to remember these matters
at all times after my death” (2 Pet 1:13-15; GNB).
Peter, knowing that he, as a disciple of the Lord of Hosts, was to “seal the OT” and “bind up the NT” says that he will make sure that the true Christians will have always “a way” to remember the true faith.
Paul Compiles the New Testament
“Only Luke is with me. Get Mark and bring him with you, because he
can help me in the work. When you come, bring my coat that I left
in Troas with Carpus; bring the books (OT) too, and especially
the ones made of parchment (the New Testament writings)” (2 Tim
4:11, 13).
There is an “urgency” in this command to Timothy. Luke is already with Paul. He asks Timothy to get Mark and bring him with Timothy to finish the work.
(It is significant that both Luke and Mark, as the Gospel writers, are together with Paul just before his death to assist him in the work!)
Second epistle of Timothy is Paul’s last epistle. He says he is about to be “poured out” (verse 6), that is, about to be martyred. So, as another disciple of the Lord of Hosts, he urgently needs to compile the NT. He already knows that Peter has edited the NT books. Now he wants to finish the final editing together with Mark and Luke and entrust the same to Timothy (may be to be handed over to Apostle John later) before his death.
What Coat
The Greek word for “coat” is “phelones”. Adam Clarke commentary:
“Τον φελονην is by several translated bag or portmanteau; and it
is most likely that it was something of this kind, in which he might
carry his clothes, books, and travelling necessaries.”
I thank Ray Butterworth for giving me the invaluable information regarding the Peshitta Bible in Syriac (an ancient translation) which translates the “coat/cloak” as a bookcase:
“When you come, bring the bookcase and the books that I left in
Troas with Qarpus, especially the parchment scrolls” (emphasis mine).
(as provided by Ray Butterworth in his answer Are verses like 2 Timothy 4:13 of exegetical and hermeneutic significance?)
Yes, Paul was not worried about his cloths at his last breath. He was concerned about the bag with the OT books and the NT parchments he left with Carpus at Troas.
The “books” are definitely the OT books. What about the parchments or “membrana”. The parchments were made of sheep skin and were very expensive in those days. These were used only for very important documents.
In the first century the New Testament writings were made on parchments. These are the documents he meant by parchments. He made the final editing of the NT together with Luke and Mark after Peter had finished his part. Both the OT and NT were entrusted to Timothy for sacred use and to be handed over to the last apostle, John.
Paul knew about the Isaiah prophecy of the completion of OT and NT among the disciples of Jesus Christ.
He, as part of the disciples of the Lord, knew he was writing the Scripture. Hence he charges:
“I charge you by the Lord that this letter be read to all the
holy brothers” (1 Thess 5:27).
“And when this letter is read before you, cause that it be
read also in the Laodicean assembly, and that you also read the one
of Laodicea” (Col 4:16).
“But if anyone does not obey our Word through the letter, mark
that one, and do not associate with him, that he be shamed” (2 Thess
3:14).
Conclusion
So, the Scripture referred in 2 Timothy 3 is both the Old Testament and New Testament writings.