This might deserve a thread of its own, but here goes ...
On 1st Timothy 3:16, we read: "All Scripture is inspired of God and beneficial for teaching, for reproving, for setting things straight, for disciplining in righteousness ..."
What Paul meant by "all Scripture"...?
The context shows what Paul was talking about. On v.15 he mentions tells Timothy that " from infancy you have known the holy writings , which are able to make you wise for salvation". If Timothy was in his early 30's when he was appointed by Paul as an elder in Ephesus, then Timothy was born circa 29 CE, that is, well before Jesus' death, and therefore, the "holy writings" that he has known since his infancy were the OT. That's the "all Scripture" that paul refers to on v.16.
Since Paul was talking about the OT, saying that it was "useful" [Greek: opheleo, meaning: profit, help, benefit] for "teaching". Shall we conclude that Paul was endorsing the OT as a source of doctrine for Christians?
No. Here's why. The context on v.15 shows that Paul considered the OT useful because they "are able to give you wisdom for salvation through faith in Christ Jesus." The salvation doesn't come from observation and obedience to the OT. Salvation comes through faith in Jesus. However, the OT can become "useful" and "helpful" to the Chrstian [in particular, the Christian with a jewish background, as it was Timothy's case], to gain wisdom to recognize the Messiah as Jesus.
While Paul described the OT Scriptures as "useful / profitable", he didn't describe them as "authority". However, he described the apostleship as "authority that the Lord gave us" (2 Corinthians 10:8) [Strong's #1832, exesti, meaning: lawful], thus tracing his authority directly from Jesus Christ.
Eden