We are not to assume that the Holy Spirit first came into being on that day at Pentecost, any more than we are to assume that Jesus first came into being the day He was born.
The Spirit of God has been at work in men’s hearts from the beginning of time. The Old Testament has many references to the Spirit of God, for the Hebrews were always aware of God’s presence in their lives. The Hebrew people never conceived of a doctrine of God as being a Trinity as the Christians have, because they were not aware of the Messiah being in existence. The Hebrews had no doctrine of a Dual God either, although they were certainly aware of God as a Spirit who entered their hearts.
The Jews of Jesus’s day strongly emphasized the unity of God, and this emphasis carried over into the early Christian church. The result of this was that some early Christians ruled out the personal distinctions of the Godhead altogether, and others failed to render full justice to the second and third persons in the Trinity.
Tertullian was the first Christian theologian to coin the word “Trinity”, and work out the doctrine, but his formula was deficient because he subordinated the Son to the Father. Origen, whom we have mentioned before, went even further by saying that the Son was subordinate to the Father, and the Holy Spirit was subordinate to the Son.
The Church began to formulate its doctrine of the Trinity in the Fourth Century. The Council of Nicea declared the Son to be co-essential with the Father. The Council of Constantinople in 381 AD asserted the deity of the Holy Spirit.
There was no further development of the doctrine of the Trinity until after the Reformation. At the time of the Reformation, there were no new doctrines advanced, but there were several of the older ones dug up again. These old doctrines were on the order of subordinating the second two persons in the Trinity to God. Some of these even went back to stating that Christ was only a man, and the Spirit was only a power or influence. This thought became the forerunner of the Unitarian sect.