The Council of Nicea concluded, to condemn the gnostic gospels, Jesus & the Father are both God yet separate; however, the Council left out the Holy Spirit as equal to God, thus God the Holy Spirit.
The doctrine of the Trinity developed as a later theologic consensus. (See the Council of Chalcedon; A.D. 451)
The early Church Fathers felt no need for such official doctrine as the Scriptures mention three divine Persons; thus God's self-evident nature revealed the Trinity.
As previously stated, the Council of Nicea didn't address the triad. Nicea didn't address the Holy Spirit as an eternal Person equal to God and God the Holy Spirit; therefore at best, the Council presented a duality.
The doctrine of the Trinity, as we know it today, evolved as an Orthodox Church teaching.
Most notably, some of the Church leaders, Athanasius of Alexandria & his mentor, Patriarch Alexander. They took up the matter of the Holy Spirit's divinity, too.
The Basis is Threefold
1). Christ was with God in the beginning. (John 1:1).
2). 30 I and my Father are one. (John 10:30).
3). Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: (Matthew 28:19).
The early church didn't thoroughly understand the Holy Spirit's role in the Trinity, or the doctrine of the Godhead as an orthodox theology until later.
What the early Church Fathers did understand consequences for the unforgivable sin as lending credence to the transcendent importance of the Holy Spirit; thus equated the Holy Spirit as the third Person of the Trinity. After all, the evidence supported the claim.