Today we honor St. Gregory, the 3rd century bishop of Nazianzus and St. Basil. They had been fellow students in Alexandria and Athens; and then, Basil’s sister, Makrina invited them to stay at her estate where they could prayerfully put together all that St. John and St. Paul wrote about Father, Son, and Holy Spirit.
Along with Basil’s brother Gregory of Nyssa, they were the Church’s first theologians. Centuries later St. Anselm would define theology as “Faith seeking understanding.”
Those three, with Makrina feeding them and heating their rooms, tried digesting all that Paul and John wrote about the Father, Son and Holy Spirit.
The way they understood the Scriptures, the mind of God from all eternity was occupied with a mental picture of himself. That mental picture is what St. John called the Word that was with God from the beginning. The unchanging loving relationship between the two Persons was the third Person, the Holy Spirit.