In the early days of the church, the sacraments were often referred to as "mysteries" and for good reason. Mystagogical catechesis was a revelation of those mysteries. Baptism is a good example. If a pagan were to stumble into church to witness a baptism, then he would be utterly confused by only letting his eyes educate him as to what is taking place.
Baptism is a mystery. It is a gift of God, and faith knows what it is. The eyes of faith recognize the blessing of baptism, but the eyes of the body are far too weak to discern the mystery apart from teaching.
Concerning infant baptism it is simple enough to cite such passages as Acts 2:38-39, "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call." This is often referred to as "proof texting." The problem with proof texting is someone from another denomination will try to tell you that "children" in Greek can be interpreted as the "young in faith."
Mystagogical catechesis, to over-simplify, is different. It gives you the whole history of God's people, from Old Testament to New Testament and interprets it all in a Christological and Sacramental way. So, mystagogically speaking, how might we hear a preacher like St. Cyril of Jerusalem tackle such a topic as "why infant baptism?" Read on.
In the Old Testament one finds much to ponder concerning water. Here are but a few examples. In Genesis 8, Noah is in the Ark with his family and the animals. The world is flooded by God, killing the rampant and out of hand world of sin. Those saved are the godly, eight in all. Being saved through water they embark on a new road which sounds a lot like what happens when the world was created. Genesis 9:1-3 is almost a repeat of what God says in Genesis 1:26 about the animals being there for the benefit of man. God even indicates that Noah will rule the beasts of the earth, just as is said about Adam. This is likened to Noah passing through water and embarking on a new road, a new life. This whole account carries with it the notion of a "new creation."
What is more pointed, perhaps, is what happens in Leviticus. In Leviticus 8:6, "Moses brought Aaron and his sons and washed them with water." Exodus 30:20 tells us why--"When they go into the tabernacle of meeting, or when they come near the altar to minister, to burn an offering made by fire to the Lord, they shall wash with water, lest they die." The wash basin was located in the outer court, outside the inner court and holiest of holies. Translate this into New Testament fulfillment--If you want to enter God's presence, then you must be washed clean. This brings to mind Christ's words to Nicodemus, "Most assuredly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God." The Holiest of Holies in the Old Testament is where God's presence dwelt. It was to be likened to the "kingdom of God." Hence, the priests had to wash.
In another instance, Joshua 1:2-3 shows the exchange of the prophetic office from Moses to Joshua. Moses dies and Joshua takes over. The first thing Joshua does is he leads Israel to pass through the Jordan River. Passing through water, they enter the holy land, the land of paradise. Joshua 3:7-8 points out that God does not begin to exalt Joshua in the eyes of Israel until the day they pass through the Jordan. Joshua begins his new road and so does Israel as they pass through water at God's command.
In 2 Kings 2:6, Elijah goes to the Jordan River to pass through it in order that he may be taken up to heaven. Elijah and Elisha pass through the Jordan (the River is parted). After Elijah is taken up, Elisha is given the promise of a double portion of Elijah's spirit. So, in verse 14 Elisha parts the water and crosses over the Jordan. Elisha begins a new road after passing through the Jordan and the people recognize, saying, "The spirit of Elijah rests on Elisha"(2 Kings 2:15). Elisha, passing through water, becomes new.
One of the best images which gives the picture of baptism in the Old Testament is that of Naaman in 2 Kings 5. Naaman, a hardened, Gentile war commander has leprosy, a skin disease. One of the things about a successful war commander to keep in mind is that he probably has weathered skin. He probably has scars and signs from past battles. Naaman comes to Elisha to be healed of leprosy, and he is told to wash in the Jordan seven times. Naaman becomes furious because he was expecting a "big show" to accompany this healing and is disappointed that washing in the muddy Jordan was the command. At the prompting of his servants, he obeys Elisha's command. What happens tells it all.
Naaman's skin, we are told, was not only cleansed of leprosy but it was restored like the flesh of a little child. In other words, this washing, in a sense, gives Naaman a new birth. His skin was better than before he had the leprosy. God's love heals wounds and goes even further than what we expect. Keep in mind that this is not baptism because Christ had not instituted it, but it gives the church the understanding of a "new beginning, a new creation, a new road." What we find after Naaman comes out of the water is that he goes back to Elisha and confesses, "I know that there is no God in all the earth, except in Israel..."(2 Kings 5:15). And Naaman says that he will no longer "offer either burnt offering or sacrifice to other gods, but to THE LORD"(verse 17). Passing through water, Naaman believes AND worships in a way different from the world (think historic liturgy). He went from the perspective of demanding his own way of receiving God's gifts to a different mindset, which consisted of submitting to the Lord's ways in worship, life, and confession.
It is fitting, then, that John the Baptist comes baptizing with a baptism of repentance. Baptism marks the end of the Old Testament and the beginning of the New Testament (But, then again, so does John). But, one may ask, "why does Jesus need to be baptized by John? Jesus doesn't have sin. He doesn't need to repent." True.
Where does Jesus go to be baptized? The Jordan River. Here comes the fulfillment and the answer to all those Old Testament occurrences through water and in the Jordan River. John tries to prevent Jesus from being baptized, but Jesus says, "Permit it to be so now, for thus it is fitting for us to fulfill all righteousness"(St. Matthew 3:15). What does it mean to "fulfill all righteousness?" The righteousness that Jesus is talking about in this writer's opinion is the pattern of the Old Testament. It is the priests washing before entering the holiest of holies. It is Joshua, Elijah, Elisha and even Naaman all wrapped up in Jesus. Jesus is saying that the Old Testament and all of the water occurrences are indicative of the pattern of God's way. Even Jesus observes the practice of passing through water(This emphasizes Christ's humanity.) As we know, Christ's being baptized is the beginning of His road to the cross, certainly running parallel to the Old Testament accounts listed above.
Just as the Levitical priests had to wash in the basin before enter the tabernacle, so today people become "spiritual priests" through baptism and are granted the right to enter God's presence as holy only through Christ's covering which comes through baptism.
Hence, if a person wants to become a Christian, be forgiven, and live a new life in Christ, then it only naturally follows that he or she must be born of water and the Spirit (St. John 3:5). Whether it be a child or an adult who comes to faith, it is fitting that they, like all those cloud of saints who have gone before us, should pass through water and the word at Christ's command, beginning a new road and becoming a new creation (2 Cor. 5:17; Galatians 6:15). Having the blessings of Christ's death and resurrection poured into the water and at His command the person receives something much more--the Holy Spirit, the forgiveness of sins, and the covering of Christ which stamps us as Christ's own.
In the big picture, from Old Testament to New Testament, it makes sense that St. Peter would say that regardless if the person be young or old, he should repent and pass through the precious waters of baptism, both, to receive the gift of the Holy Spirit, but to also embark on the new road that belongs to the Lord. What a holy mysterion (mystery) is baptism! Thanks be to God.