St. John the Baptist and Forerunner came out of the Wilderness of the Holy Land preaching that the Kingdom of Heaven is at hand. We should look at this prophetic statement carefully. He does not say that the Messiah or Christ is coming. He does not say that the King of Israel was born and has now reached the age of maturity (which at that time was considered roughly about 33 years of age). He says the Kingdom of Heaven is at hand. In the Gospels the phrases Kingdom of God and Kingdom of Heaven are used interchangeably, with Heaven as sort of reverent substitute for the word God as there is still to this day a Jewish avoidance to saying anything that resembles the name of God (Even today the word God is written by some Rabbis today as "G-D" out of reverence). It is interesting to note that the word in Greek βασιλεία that is translated as Kingdom here is universally agreed on by Biblical Scholars and many Orthodox Christian Theologians does not in fact mean Kingdom but is a noun meaning the reign or rule of a Ruler. In the New Testament, there is a divergence in terms of choice by the Gospel Writers and the Epistle Writers. St. Paul used the term Χάρις and the Gospel Writers use βασιλεία, but they are both speaking about the same thing: The Divine Uncreated Ruling Energy of God which Christ gives freely to us as a Gift as long as we are prepared to see it with the Eyes of our Heart.
Matthew 5:8 says Blessed are they who are pure in Heart for they shall see God. St. John the Theologian is no exception to this, as a Prophet he is pure in heart and thus sees God. He knows full well who the Messiah really is, he remembers deep in his Heart that when his mother Elizabeth was visited by her cousin the Virgin Mary, he leaped in his mother's womb (Luke 1:41). He leaped because he saw God in the womb of the Virgin Mary the Theotokos. In the narrative of Saturday's Gospel, he is a grown man who is a Prophet who sees God regularly.
He sees the God who has become Man in Jesus Christ, in his prophetic visions of God. So with Christ about to begin His Earthly Ministry, He sees the Divine Uncreated Ruling Energy of God, the βασιλεία (which is a part of God-- God is both Essence and Energy), coming. He sees that Christ is God working to bring the Tongues of Fire of Pentecost, which is this same Ruling Energy of God, into the world (Luke 12:49). Therefore St. John says repent for the Rule of Heaven (God) is at hand. It is not a shock to St. John who Christ is, but it will be a shock that God Himself is asking to be baptized by Him (Matthew 3:14). He does not expect to be asked by Christ God to be baptized but He does expect that God is bringing His Light, His Uncreated Ruling Energy into the World (John 1:6-10).
May the Light of God, God’s Uncreated Ruling Energy His Grace, that St. John the Baptist would see in His Prophetic Visions, that the Apostles received at Pentecost, that we received at our Baptisms, also come and enlighten us!