The original Nicene creed doesn't affirm the doctrine of the Trinity. Read it here
How could Paul think that Christ fulfilled God's law but not think that Christ was God? And if there was no belief in the Holy Spirit, where did the tradition of Pentecost come from? What is the Paraclete?
This is where you're confused. Paul most certainly believed that Christ was/is God, and that the Holy Spirit was/is God. All the New Testament writings affirm that fact. But none of them says that "the Man Christ Jesus" or the Holy Spirit are eternally distinct from God.
God is a Spirit. The Spirit of God was/is in Jesus. The Spirit of God active in the lives of Christians (and also speaking through the prophets, conceiving the begotten Son in Mary, etc).
Can you recognize that there's a big difference between believing that "God was in Christ, reconciling the world to Himself" and this:
- But the Godhead of the Father, of the Son, and of the Holy Spirit is all one, the glory equal, the majesty coeternal. 7. Such as the Father is, such is the Son, and such is the Holy Spirit. 8. The Father uncreated, the Son uncreated, and the Holy Spirit uncreated. 9. The Father incomprehensible , the Son incomprehensible, and the Holy Spirit incomprehensible. 10. The Father eternal, the Son eternal, and the Holy Spirit eternal. 11. And yet they are not three eternals but one eternal. 12. As also there are not three uncreated nor three incomprehensible, but one uncreated and one incomprehensible (the Athanasian Creed of the 500s AD).
It's the belief in an eternal distinction of persons that makes Trinitarian doctrine non-Biblical, and contradictory to strict Monotheism. Jews and Muslims recognize this, while mainstream Christianity can only weakly claim that "The Trinity is a Mystery".