Immediately after Pentecost we celebrate the central mystery of our faith: the One, Almighty God in three equal and undivided, yet distinct Persons. We call God the “Holy Trinity” to express this truth which Our Lord taught us through His Apostles, that “He and the Father are One (and the same God)”, and that He sends the Holy Spirit upon His Church to make Him and the Father known.
Since it is only possible for God Himself to make God known, therefore the Spirit, too, is One with the Father and the Son. Son and Spirit together make known to us, as far as is possible for our limited minds, the wonderful Mystery of God as He is in Himself: Light, Life and Love.
Yet even in the Old Testament there were signs of the truth that would eventually be revealed to the Church by the Son and the Spirit. We are familiar with the imagery showing an elderly Father, a young Son, and a Dove representing the Spirit, which draws attention to the differences between the three persons of God.
~ Fr. Paul Dobson.