Today is Theophany Eve according to the Old Calendar, also known as Krstovdan, the day of the Holy Cross in the Serbian tradition, but I think this refers to the fact that catechumens were traditionally baptized on the eve of two major feasts in the early Church: Pascha and Theophany. Hence, today is known as Krstovdan (in Serbian the word baptism is krstenje, coming from the word for Cross=krst).
The following I found on the Catholic blog Vox Nova (here):
For the Christian East, the baptism of Christ is commemorated as one of the most important liturgical feasts of the year. It is certainly one of the most theologically rich celebrations, filled, as it is, with the revelation of several important Christian mysteries. There is much one can say about it – if one looks carefully, one could find the full message of the Gospel revealed upon this one day. For baptism, as St Paul tells us, unites us with the death and resurrection of Jesus; the baptism of Christ, therefore, must present this mystery to us, and with it, all that contained in the Paschal story. Easter is a Trinitarian event, and so, without surprise, the same must be said about Christ’s baptism; indeed, because the Trinity is presented to us in a special way at Christ’s baptism, it is generally known as Theophany, the “appearance of God.” All three persons of the glorious Trinity are present, even as they are involved with Christ’s work on the cross. But, humanly speaking, since the Trinity is revealed at Christ’s baptism, it is said that it is at this point that we are shown the greatest mystery of all, the Tri-personal nature of the Godhead.
“When Thou wast baptized in the Jordan, O Lord, the worship of the Trinity made its appearance. For the voice of the Father bore witness to Thee when He called Thee His beloved Son. And the Spirit in the form of a dove confirmed the truth of the word. O Christ our God, Who hast appeared and hast enlightened the world, glory to Thee! ” (Troparian of Theophany).
Trinitarian theology requires one to understand the proper relationship between the Father, Son and Holy Spirit. The Scripture reading for the day presents the foundations for this theology. “Then Jesus came from Galilee to the Jordan to John, to be baptized by him. John would have prevented him, saying, ‘I need to be baptized by you, and do you come to me?’ But Jesus answered him, ‘Let it be so now; for thus it is fitting for us to fulfill all righteousness.’ Then he consented. And when Jesus was baptized, he went up immediately from the water, and behold, the heavens were opened and he saw the Spirit of God descending like a dove, and alighting on him; and lo, a voice from heaven, saying, ‘This is my beloved Son, with whom I am well pleased’” (Matt 3:13 – 17). The philosophical and theological implications of this event would slowly emerge, and it would only be after the development of technical vocabulary that the Trinitarian nature of the Godhead could be properly signified (but not captured) by human language. The notion of personal distinctions, so easily accepted by today’s subjective world, was a difficult concept to develop, and only came out of the Trinitarian debates of the ancient world (this shows us the stamp of Christian theology on contemporary life: even for those who have abandoned the Christian faith accept the notion of person established by the Christians as a result of their debates about the Trinity). A son shares with a father the same nature, but they do not share the same subjective dimensions – and so, the Father and the Son, revealed at this event, are eventually said to be homoousios, sharing the same nature, but having subjective distinctions which make them distinct, one from another. These subjective distinctions do not make for objective division – a fact which, when reflected upon, counteracts the mistaken tendencies of different ages. In the ancient world, it was easy to accept objective unity, and subsume subjectivity to it, diminishing the person as a result; in the modern world, the reverse tends to be the case – the person is pushed forward at the expense of objective unity, ending up with subjective individualism that allows for no real substantial human nature to unify us (with human nature is seen merely as a construct to be deconstructed). Yet, we need to realize both dimensions in order to find our proper place in the world. It is only in this fashion that we can also overcome the errors of both monistic and dualistic world views. While we can accept neither of them, we must accept the truth contained in both, meaning, we must accept a third, mediated position between the two, the position exemplified by the Trinity: subjective distinction does not have to end up individualistic (not three Gods), even as objective unity does not have to destroy personal subjects in order for such unity to exist (thus, modalism is to be rejected).
Since Trinitarian debates became the foundation for our understanding of the person, it is not surprising that in this Trinitarian event, not only are we given a revelation of the Divine Trinity, but we are also given a revelation about the human person as well. The relationship between John the Baptism and Jesus Christ presents to us the kind of loving-humility that one will find in perfected humanity. Both had to humble themselves, one to another, in different ways, for different reasons. John the Baptist knew his work was a perfectly human work, with a human end – but he also knew the time would come that God’s grace would be added to the work of baptism, and that the one who would transform the work of baptism into a divine work would be the messiah. It is for this reason he knew he was not Christ’s equal, and was rather confused when Christ came to him asking to be baptized. How could John, whose mission it was to prepare the hearts of the people of Israel for the work of Christ, dare to wash the messiah in his baptism, when he understood everyone needed the baptism which the messiah would later bring? And yet – that is what Christ expected of him. John found himself at a point of crisis, and came out on top – he was able to come to the correct answer, and, despite his confusion, baptize Christ in the Jordan.
John was right to believe that there was something special about the messiah (though the theology of the incarnation was probably not known to him); but only when he came to baptize Christ would he fully understand that the power of authority is known in and through humility. Jesus was the messiah and therefore had far greater authority and glory than John, but Jesus, in and of his human nature, was John’s equal (homoousios). Personal glory does not diminish the natural equality of one human person to another (a dignity guaranteed by the fact that there is only one human nature). Nonetheless, the equal dignity of human persons must not be used to eliminate personal distinction and the authority which comes from these distinctions. There are variations of glory to be found in humanity, and these differences should be respected; but glory, and the authority associated with it, is had and known through humility, through servitude (even as divine glory is known through its kenosis). Jesus’ human glory was fulfilled in his role as the messianic servant; this required him to bring himself to John, to put himself under John, showing that he freely followed the Old Covenant (John, being the last of its prophets). But John, being a prophet of God with real authority given to him, had to understand his proper place in the world, and the humble service he was called to by God for the sake of humanity; that humility was tried when he found himself being asked to baptize Jesus. Would he follow Jesus’ petition (which he should do) despite his lack of understanding why he was being asked to do it, or would he put his own understanding above Jesus’ petition, and deny Jesus his baptism? It is only when we bow our will to God, despite our understanding, that we find God will be able to take our work and perfect it – and our understanding of it (an ancient interpretation of the fall of Satan was that he was given a similar crisis, but, unlike John, he relied upon his own understanding, causing him to disobey God’s command).
John’s glory was revealed in his humility, and in that glory he showed us what is expected of the human person: humility in action. Looking not for glory for ourselves, we must be humble about what we do – leaving room for divine grace to take our work and perfect it. Pride can close us in upon ourselves; false humility can do so as well, since it will be used as an excuse to prevent us from trying to do what it is we feel called to do “I would do it, Lord, except I am not worthy, find someone else.” The reason we say this is our pride does not want to see failure; true humility, on the other hand, will not care – it does not look for success for ourselves, and so will open us to the grace needed for success, even as John’s humility, open to Christ’s will, allowed him to act and baptize Christ. But to what effect? What was the purpose of this event beyond this interesting human drama? What did Christ want to accomplish?
“Make ready, O Zebulon, and prepare thyself, Nephthali. River Jordan, stop flowing and leap for joy at the Lord’s coming for Baptism. Rejoice, O Adam, with our first mother; hide not yourself as in Paradise of old. For seeing us naked He has appeared, to clothe us with our first garment. Christ has appeared, to renew the whole of creation” (Troparion, Forefeast of Theophany). “The river Jordan was once turned back by Elisha’s mantle when Elijah was taken up, and the waters were divided hither and thither. The watery path became dry for him as a type of baptism whereby we cross the flowing stream of life. Christ has appeared in the Jordan to sanctify the waters” (Troparion, Eve of Theophany).
In a fallen mode of being, the world and all that is within it is divided into warring parts, with different fragments of itself fighting against other such fragments, one against another, in what might appear to be a war which will only end when reality manages to finally annihilate itself. Human society mirrors these destructive tendencies of creation, where various persons temporarily join together to fight others, until at last their own fragile, social dimension is shattered and new alliances and hatreds form, making former allies enemies, ready to work for each other’s mutual destruction. But humanity also makes war, as it were, upon the earth; the world and all that is within it is seen as a hostile force to be overcome (less humanity is itself the one who is subjugated). The world is experienced as a terror, its powerful forces capable of great destruction – and without any harmonic, integral unity with humanity, they are open to be manipulated by the forces of chaos and destruction (the daemonic elements of creation); thus humanity finds itself not only at war with the world, but also with the forces of chaos (personal or not, spiritual or not) that seek to control the world for their own hateful purposes. St Symeon the New Theologian faithfully presented this bleak portrait of the world after the fall of Adam in his many works, such as when he wrote : “Do you see how the earth now cursed and deprived of its spontaneous germination, received the transgressor? […] Therefore, indeed, when it saw him leave Paradise, all of the created world which God has brought out of non-being into existence no longer wished to be subject to the transgressor. The sun did not want to shine by day, nor the moon by night, nor the stars to be seen by him. The springs of water did not want to well up for him, nor the rivers to flow. The very air itself thought about contradicting itself and not providing breath for the rebel. The wild beasts and all the animals of the earth saw him stripped of his former glory and, despising him, immediately turned savagely against him. The sky was moving as if to fall justly down on him, and the very earth would not endure bearing him upon its back.”  Many of the patristics believed that the fall of humanity left the world open for demonic assault – allowing for Satan and his followers to use and abuse the powers of the earth for their own desires. The world not only became a natural terror, but a supernatural one which left humanity helpless to the powers of evil. The more unbalanced humanity became, the more self-destructive humanity became, the more the world lost its center, and the more the world took on the natures of chaos and fought harshly against its former steward. It is in this light that Christ is seen to restore not only fallen Adam (and humanity in Adam) but the whole of the world – preparing the way for human salvation by sanctifying and purifying the elements of the world themselves. “For the creation waits with eager longing for the revealing of the sons of God; for the creation was subjected to futility, not of its own will but by the will of him who subjected it in hope; because the creation itself will be set free from its bondage to decay and obtain the glorious liberty of the children of God. We know that the whole creation has been groaning in travail together until now; and not only the creation, but we ourselves, who have the first fruits of the Spirit, groan inwardly as we wait for adoption as sons, the redemption of our bodies“ (Rom. 8:19 – 23). The baptism of Christ was accomplished, in part, for the purification of the elements of the earth, beginning their transfiguration in Christ – the waters of the earth are sanctified in and through the waters of Jordan, even as Adam is sanctified in and through the death and resurrection of Christ. The lost harmony of the earth found a new center in Christ, one which will allow for their eschatological reintegration; the demonic centers of chaos, the demons, prideful in their power, found their authority over the earth overcome by the humble obedience of Christ: humility is stronger than pride, love is stronger than hate.
“Thou hast appeared today to the world, and Thy light, O Lord, has been signed upon us who with full knowledge sing to Thee. Thou hast come, Thou hast appeared, O Unapproachable Light” (Kontakion of Theophany).
Having sanctified the waters, Christ has made them into a vehicle of grace. Baptism is the sacrament of initiation -or, as the ancients would say, the sacrament of enlightenment. We are opened up to grace: in going down into the waters, we die with Christ, and being raised up out of them, we are “born again” – experiencing the resurrection of Christ in our life, and with it, we are opened to theosis, whereupon we will experience (through Christ) the divine life. Baptism is Trinitarian because the new life is Trinitarian – our spiritual life can only be accomplished in and through our complete interaction with all three Divine Persons. The light of truth opens us to the light of the Trinity; in our baptism, we are shown the glory of the Trinity, even as Christ’s initial baptism presents the persons of the Trinity to us. “What are we to learn and be taught by this? To purify ourselves first; to be lowly minded; and to preach only in maturity both of spiritual and bodily stature.” Baptism is about glory in humility, authority in servitude, power in self-abasement. As Jesus said, we must die to ourselves in order to live.
 St Symeon the New Theologian, The Ethical Discourses Vol. 1: The Church and The Last Things. Trans. Alexander Golitzin (Crestwood, NY: St Vladimir’s Seminary Press, 1995), 28-9.
 St Gregory Nazianzus, “Oration on the Holy Lights,” NPNF 2 (VII),357.