Here is part of a brilliant study of II Timothy by David Guzik 2018
3. (16-17) Timothy must continue with confidence in the Holy Scriptures.All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work.
a. All Scripture: This indicates more than the Hebrew Scriptures. If Paul meant the exact same thing here as what Timothy learned as a child, he might have said “Those Scriptures” referring back to verse 15, or he might have just repeated the exact phrase, “Holy Scriptures.”
i. Paul changed his wording here because he recognized that what God uniquely brought forth from the apostles and prophets in his time was also Scripture; it was also the God-breathed word of God. This included what he and others knew was emerging as the written form of the foundation of the apostles and prophets mentioned in Ephesians 2:20.
ii. This would fulfill the promise Jesus made that the Holy Spirit would speak to the apostles and lead them into all truth.
iii. There is no doubt that Paul thought this way – knowing that God was bringing forth a New Testament through the apostles and prophets of the first century.
· Paul commanded the public congregational reading of his letters, as would be done with the Hebrew Scriptures (Colossians 4:16, 1 Thessalonians 5:27).
· Paul called his own message the word of God (1 Thessalonians 2:13).
· In 1 Timothy 5:18, Paul combined a quotation from the Old Testament, and some words of Jesus recorded in Luke 10:7 and he called both of them “Scripture.”
iv. Paul wasn’t the only one who thought this way. 2 Peter 3:15b-16 indicates the same idea, especially when Peter included Paul’s writings under the heading, Scriptures.
v. All this reminds us that even in Apostolic times, they were well aware that God was bringing forth more Holy Scripture, just as Jesus promised, just as Paul described, just as Peter understood.
See the whole study HERE.enduringword.com/bible-commentary/2-timothy-3/