A man was sent. By whom? By the divine Word, whose forerunner he was. To go before the Lord was his mission. Lifting up his voice, this man called out: The voice of one crying in the wilderness!
It was the herald preparing the way for the Lord's coming. John was his name; John to whom was given the grace to go ahead of the King of kings, to point out to the world the Word made flesh, to baptize him with that baptism in which the Spirit would manifest his divine Sonship, to give witness through his teaching and martyrdom to the eternal light.
John Scotus Erigena (AD 810-877), Hom. sur le Prologue de Jean, 15: SC 151, 277
Erigena was an Irish theologian whose theological synthesis was the first of its kind in the Middle Ages.
Of note is the distinction Erigena makes between the manifestation of Christ's sonship by the Holy Spirit at John's baptism and his actual possession of that sonship. In more technical terms, Erigena recognizes the crucial distinction between the internal (ad intra) versus external (ad extra) life of the Trinity. The triune God's processions in Himself are not strictly identical to His missions in the actualized work of redemption. There is an internal procession of the Son from the Father and a spiration of the Holy Spirit from the Father and (i.e., through) the Son, which have been and may be manifested in various ways in the mission of redemption. In a word, processio (proceeding) is not always identical to missio (sending).
The mission under discussion here is that Christ's Incarnation infused divine grace into such acts as baptism, thus giving them new and lasting spiritual efficacy in the establishment of His Kingdom. By His word, His nature, He has infused otherwise impotent water with grace - baptism - in order that humans, by that baptism of faith, would be similarly transformed and empowered to be living vessels of grace. The sacraments are the lasting, effectual signs of and means by which we experience the fact that God, in Christ, by the Holy Spirit, became truly "humanized" in order that man, in Christ, by the Holy Spirit, could become truly divinized. (The name of the former process is kenosis [see Php 2]. Eastern Orthodoxy refers to the latter process as theosis.) Christ was recognized externally as the Son of God by His baptism. The Holy Spirit testified in time, publicly, to what Christ had possessed from eternity, privately. We, by contrast, are actually made sons of God by baptism and have the internal testimony of the Holy Spirit (see Galatians 3, Romans 6-8 and 1 Peter 3:18ff.).