The two pillars of Christianity are the Holy Trinity and Christ's Incarnation. The Trinity is three distinct persons — Father, Son and Holy Ghost — all sharing in the same divine nature. During Christ's Incarnation, the divine Second Person of the Blessed Trinity united with unsullied human nature in Mary's womb. While the Incarnation involves one "who" and two "whats," the Holy Trinity involves three "whos" and one "what."
Each person of the Trinity shares equally in the one divine Godhead but is distinct in origin. God the Father is unoriginated from all eternity. God the Son is begotten solely from the Father. God the Holy Spirit is the bond of love between the Father and the Son and is thus said to proceed from them both.
The great Doctor of the Church, St. Augustine, explained these two processions as spiritual processes of knowing and loving. He taught that God the Father, contemplating Himself from all eternity, begets the perfect understanding of Himself, which is the Divine Word — Jesus. This follows the teaching of St. John in his Gospel, John 1:1 which reads, "In the beginning was the Word, and the Word was with God, and the Word was God."
Moving to the spiration of the Holy Spirit, St. Augustine explains that the Third Person of the Blessed Trinity proceeds from the bond of love between the Father and the Son. This agrees with what St. John teaches in 1 John 4:16, "God is charity: and he that abides in charity abides in God, and God in him."
The Old Testament gives a foreshadowing of the Trinity when in Genesis 1:26 God says, "Let us make man to our image and likeness." Before ascending, Christ explicitly mentions the Trinity in Matthew 28:19, "Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son and of the Holy Ghost."
Delve into this great mystery of God in season one of Armor of God—God In Himself: The Holy Trinity.