Oh?
Then just what CAUSED so many to join this new 'teaching' at Pentacost?
That is a very different question than that of the doctrine of the Trinity.
Of course, the Church has always had the Holy Spirit and as the years went on the writings of the Apostles.
At Pentecost and for many years there was only the verbal preaching of the Gospel. The first written words of the NT did not come for nearly two decades following Christ’s resurrection.
Let me say this regarding your question.
When Peter stands to speak after the coming of the Holy Spirit he does not say that Jesus is God, the second person of the Holy Trinity. He says that God has made this Jesus, Lord and Messiah!
The Apostles did not know everything from the beginning. That is why Jesus promises to send the Holy Spirit to lead them to all truth. Not everything was perfectly clear as yet to them, just as when Jesus spoke of the Eucharist, it was not clear how He could give them His flesh to eat and His blood to drink.