***I am quite certain that +Paul did not believe in the Holy Trinity because his gospel (as he called his preaching) does not reveal it. ***
I am confused. Paul continually talks of the Spirit. Just as he talks of the Father and the Son. In fact, Romans 8 is all about the Spirit of God.
St. Paul refers to the Spirit of God in the OT sense, as a power of God and not of a divine Hypostasis. It is clear that he does not speak of the Spirit in the trinitarian sense.
St. Paul places God above Christ when he says on numerous occasions that God raised Christ. Christ is not subordinate to God, which is why the Nicence Creed specifically says "He rose on the third day..." in other words, God raised Himself.
There is also a reference in the NT when Jesus says that the "Father is greater than I" which the Apostolic Church interprets as Christ speaking in His human nature, and some Christian groups as meaning that Christ is a "lesser" God.