Thank you for the biblical question, “When Paul wrote 2 Timothy 2:15, what was considered to be the word of truth at that time?”
Here are some thoughts for one’s consideration of the Scriptures at the time of writing the Second Epistle to Timothy:
A. Saul of Tarsus as a Pharisee accepted the three fold division of the Old Testament.
The three fold division of the Jewish Scriptures are as follows: the Torah or Law, the Neviʾim or the Prophets, and the Ketuvim or the Writings.
Luke 24 says this, “25 He said to them, “How foolish you are, and how slow to believe all that the prophets have spoken! 26 Did not the Messiah have to suffer these things and then enter his glory?” 27 And beginning with Moses and all the Prophets, he explained to them what was said in all the Scriptures concerning himself” (NIV). Citations are in the NIV.
In the same chapter, Jesus said this, “4 He said to them, “This is what I told you while I was still with you: Everything must be fulfilled that is written about me in the Law of Moses, the Prophets and the Psalms.” 45 Then he opened their minds so they could understand the Scripture.”
The three fold division of the Old Testament Scriptures were accepted by the Teachers of the Law. Since Saul was taught under Gamaliel, it is most likely that he accepted the three fold division of the Scriptures with its books.
B. Jesus commanded his disciples that they are to teach all that he commanded them.
Matthew 28 says this, “19 Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20 and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age.”
Acts 2 says this, “42 They devoted themselves to the apostles’ teaching and to fellowship, to the breaking of bread and to prayer. 43 Everyone was filled with awe at the many wonders and signs performed by the apostles. 44 All the believers were together and had everything in common. 45 They sold property and possessions to give to anyone who had need. 46 Every day they continued to meet together in the temple courts. They broke bread in their homes and ate together with glad and sincere hearts, 47 praising God and enjoying the favor of all the people. And the Lord added to their number daily those who were being saved.”
The disciples of Jesus began teaching the things that Jesus had taught them. The Holy Spirit was the one who enable them to remember the teachings of Jesus.
Thus, at this point of time the Old Testament Scriptures and the teachings of the Jesus to his disciples were profoundly cherish by the believers.
If the disciples devoted themselves to the apostle’s teaching, then there was verbal transmission of the words of Jesus to the new believers. I would think that Saul of Tarsus would have come under the teachings of the Apostles before he was sent with the Letter of Jerusalem to the Gentile churches. Depending on one’s dating of the Gospel of Matthew, Mark, John, and Luke, the verbal transmission was at some point before 70 CE, the words were written. Dr. Luke states that at the time of his writing of his letter to Theophilus, that others had already begun writing the life and or words of Jesus. (Luke 1:1–4).
If II Timothy was written around 66/67 under Emperor Nero, then 30 years has transpired. I would think that by that time, the teaching of Jesus was quite well-established by the believers. With the aging of the Apostles or their imprisonment, it is likely the Gospels were written as a permanent record of the words and work of Jesus.
The Apostle Paul’s letters to the Gentile churches became cherished or treasured for his wisdom and teaching in dealing with the needs of the Gentile churches. As the Gentile churches received the Letter from Jerusalem in Acts 15, I would think that the Gentile churches hope to have other letters to teach them how to deal with their beliefs and practices. God used the Apostle Paul to meet the needs of the early church in his letters which has become canonized or accepted as Scripture by the early Christians.
Summary: The Jewish Scriptures and the teachings of Jesus were considered Scripture by the early Christians. The letters of the Apostle Paul was later considered “Scripture” for his apostolic authority and his teaching within the Christian churches and believers.
-Kingston