God is One, yet God is three. This fundamental reality of the nature of God and apparent contradiction has been a hotbed for debate since Christianity first began. How can God be one while also being three? How can Jesus say, “The Father and I are one” (John 10:30), yet still be possible that the Father and the Son are distinguishable? These are questions which must be thought about and struggled with for any Christian. One substantial question pertaining to the incarnation is, if the Godhead is a substantial unity eternally existing in three persons, how can one person become incarnate? The question of how divinity and humanity come together in the incarnation is one which we cannot come to fully comprehend, yet we will seek to better understand.
Historically, the church has confessed the substantial unity of the Godhead. The one God who eternally exists as three persons. This existence is then described in the terms of paternity (unique to the Father), generation or filiation (unique to the Son), and spiration or procession (unique to the Spirit). That is to say, the Father has always been the Father, the Son has always been the Son, and the Spirit has always been the Spirit. There never was a time when one of the persons of the Trinity did not exist. In addition, the Father has always generated the Son, and the Spirit has always proceeded from the Father and the Son. These three things have been termed ‘eternal relations of origin.’ This reality is the only distinction between the persons of the Trinity ad intra, which is seen most clearly in how the persons of the Trinity interact with creation.
While the aforementioned relations of origin describe the relations within the Godhead, these relations are based upon the manner in which the Godhead is revealed in the world. Jesus, the Son of God, who is one with God and who is himself God (John 1:1-2) became enfleshed and dwelt in the midst of humanity (John 1:14) as one sent from God. The temporary mission of Jesus reveals the eternal relations of the Godhead. The unbegotten Father eternally generates the Son, a relationship which is revealed fully in the world in the incarnation of the Son of God. While the Father always generates the Son and the Son is always begotten from the Father, this relationship is seen clearly in the incarnation when the Son, not the Father, is generated within creation.
In a similar manner, Jesus sends the Spirit, the Helper (16:7), who indwells believers. The Father and Son together spirate the Spirit (John 15:26) who proceeds from them in eternity. This spiration is seen in the world by the Spirit being breathed out from God and into His chosen people. As the Father breathed life into Adam in the beginning, so too the Father and Son breathe the Spirit into His sons and daughters.
Thus, while the eternal relations of origin are the only distinctions within the substantially united Godhead, these distinctions help us to better understand the nature of both the incarnation and the indwelling of the Spirit—the persons of the one God are enacting their relations in a manner by which we can come to better relate to and worship this God.