Saint Gregory the Theologian refers to this great time of celebration in the Church as the "Day of Light."
He says,
For the Holy Day of Lights, to which we have come, and which we are celebrating today, has for it's origin the Baptism of my Christ, the True Light That lightens every man that comes into the world, and effects my purification, and assists that light which we received from the beginning from Him above, but which we darkened by sin. The image of Christ as one who brings light to the world can be a powerful one if we realize that without Him we are in darkness. It is His light that lights the path to His Kingdom, to union with God. Without it the path is impossible to find and we are left wandering in darkness seeking what we cannot find.
Saint Gregory says,
Therefore listen to the voice of God, which sounds so exceeding clearly to me, who am both disciple and master of these mysteries, as would to God it may sound to you; I am the Light Of The World. Therefore approach ye to Him and be enlightened, and let not your faces be ashamed, being signed with the true Light. It is a season of new birth, let us be born again. It is a time of reformation, let us receive again the first Adam. Let us not remain what we are, but let us become what we once were... See the grace of the day; see the power of this mystery... This event is the establishment of the Sacrament of Baptism where we begin our healing from the sinful natured we have inherited from the fall of Adam and Eve. It is an event celebrating our renewal through our baptism, the time we became Christians with the Holy Spirit within guiding us toward a life like Him, made evermore effect as we participate in the teachings, practices and sacraments of the Church.
Saint Gregory puts it this way,
We were turned into a den of all sorts of passions, which cruelly devour and consume the inner man; but there was this further evil, that man actually made gods the advocates of his passions, so that sin might be reckoned not only irresponsible, but even divine, taking refuge in the objects of his worship as apology. It is on this day that Jesus entered the waters of the Jordan and submitted to the baptism of John to renew and fulfill it as an act to join us to God, being absolved of all our sins through the grace of the Holy Spirit.
Saint Gregory says,
Jesus goes up out of the water ...for with Himself He carries up the world ... And sees the heaven opened which Adam had shut against himself and all his prosperity, as the gates of Paradise by the flaming sword. And the Spirit bears witness to His Godhead, for he descends upon One that is like Him, as does the Voice from Heaven..., and like a Dove, for He honors the Body... By being seen in a body form... On this day the divine nature of Christ is revealed as part of the Holy Trinity. He became fully flesh in the Incarnation to renew all of mankind. Today we celebrate the beginning of His actions to help us rejoin in union with Him.
Reference: On The Holy Lights by Saint Gregory the Theologian