The only scripture the NT church possessed was the OT.
When Jesus spoke
Matthew 28:19, they knew He was speaking of
Isaiah 9:6, among others. If they simply said, Father, son and Holy Ghost, they would only have repeated what Jesus said to do and not obey what He was teaching them. "I and my Father are one."
Jesus taught them from the very scriptures they were familiar with - "And beginning at Moses and all the prophets,
he expounded unto them in all the scriptures the things concerning himself." (
Luke 24:27)
That is why Peter could say, "To Him all the prophets witness that, through His name, whoever believes in Him will receive remission of sins." (
Acts 10:43)
And why Peter could also say, "But grow in grace, and in the knowledge of our Lord and Saviour Jesus Christ. To him be glory both now and for ever. Amen. (
II Peter 3:18)
They studied from what we now call the Old Testament. Those in Thessalonica testify of what they studied and where they got their information to verify the words of the Apostles.
"These were more noble than those in Thessalonica, in that they received the word with all readiness of mind,
and searched the scriptures daily, whether those things were so." (
Acts 17:11)
ILG, when we already know these things, church leadership cannot hurt us. Disappoint, yes, but you should have already know this as you helped pastor a church. I know that was blunt, but it's true.