when the Newly-Illumined, just a couple of days ago catechumens, find out just what's happened to them.
During the Vigil, when we dunked them in the baptistry, we threw them into the great deep Catholic sea, and they'd better learn how to swim in it. Sometimes it takes a while.
In the early times of the Church, it was even more so. In those days, only the baptised were allowed to know about or witness the Sacraments. The liturgy of the Word is also called the "Mass of the catechumens" because it was the only part they were allowed to be at. In the Liturgy of St. John Chrysostom one can still hear the command "The doors! The doors!" when the catechumens and other unbaptised were escorted out and the doors closed and guarded so no unbaptised would witness or intrude upon the Sacrifice.
So, after being instructed in the Christian life and moral living, the catechumen would be brought to the Vigil not knowing what was going to happen to them, and both witness the Eucharist and receive It for the first time that night.
Then, clothed in their baptismal white garments (that they would wear all week), the bishop would instruct them about what had just happened to them, and especially about the sacraments; this instruction is called the mystagogia.
Following is the very beginning of the mystagogia as taught by St. Ambrose of Milan:
We gave a daily instruction on right conduct when the readings were taken from the history of the patriarchs or the maxims of Proverbs. These readings were intended to instruct and train you, so that you might grow accustomed to the ways of our forefathers, entering into their paths and walking in their footsteps, in obedience to God’s commands.
Now the season reminds us that we must speak of the mysteries, setting forth the meaning of the sacraments. If we had thought fit to teach these things to those not yet initiated through baptism, we should be considered traitors rather than teachers. Then, too, the light of the mysteries is of itself more effective where people do not know what to expect than where some instruction has been given beforehand.
Open then your ears. Enjoy the fragrance of eternal life, breathed on you by means of the sacraments. We explained this to you as we celebrated the mystery of “the opening” when we said: Effetha, that is, be opened. Everyone who was to come for the grace of baptism had to understand what he was to be asked, and must remember what he was to answer. This mystery was celebrated by Christ when he healed the man who was deaf and dumb, in the Gospel which we proclaimed to you.
After this, the holy of holies was opened up for you; you entered into the sacred place of regeneration. Recall what you were asked; remember what you answered. You renounced the devil and his works, the world and its dissipation and sensuality. Your words are recorded, not on a monument to the dead but in the book of the living.
There you saw the levite, you saw the priest, you saw the high priest. Do not consider their outward form but the grace given by their ministries. You spoke in the presence of angels, as it is written: The lips of a priest guard knowledge, and men seek the law from his mouth, for he is the angel of the Lord almighty. There is no room for deception, no room for denial. He is an angel whose message is the kingdom of Christ and eternal life. You must judge him, not by his appearance but by his office. Remember what he handed on to you, weigh up his value, and so acknowledge his standing.
You entered to confront your enemy, for you intended to renounce him to his face. You turned toward the east, for one who renounces the devil turns toward Christ and fixes his gaze directly on him.