A while back Huffington Post had an article concerning baptism in the time of Augustine and Ambrose. What it describes is remarkably similar to the rites of the Mystery Cults. From the article:
In early Christian communities — in fourth century Milan, for instance — it was the day when, normally, all new Christians were baptized, and it was the deepest spiritual experience most of them would ever have… things like their being stripped naked in church and then anointed with oil all over their body, or going naked into a pool where their heads would be held under water three times for “total immersion”…
Baptism then involved, for those seeking it, a long Lent of fasting and penance, during which Ambrose preached twice a day on the meaning of baptism… Ambrose said that passage through the waters of baptism was the culmination of a whole series of biblical signs and prophecies — riding with Noah through the Deluge, crossing with Moses through the Red Sea, receiving healing words from Jesus at the Jordan River and the Pool of Bethesda.
By going into the healing waters of the very pool we can see today, Ambrose taught, his flock was dying into Christ, to be resurrected with Christ on the other side of the pool, and then to be “clothed in him” as the white robes of a newfound innocence were donned…
The white flock of renewed souls sang their way into church with the hymns that Ambrose had composed and taught his congregation… At the first Mass these new Christians were allowed to join in, they heard for the first time the Lord’s Prayer (a treat that had been kept secret for them) and received their first communion of the Body and Blood of Christ…
Even then their spiritual instruction had not ended. Still clothed in their white garments, they heard a sermon a day for another week as Ambrose “debriefed” them on all that they had undergone, bringing out deeper and deeper meanings in the mystical rite of rebirth that is baptism.
So here we have a description of the Baptism practiced by Ambrose. It sounds remarkably like an initiation to a Mystery Cult. In The Golden Ass by Apuleius, he describes a lot of similar features. Initiates wore white robes:
Then came the throng of those initiated in the mysteries, men and women of all ranks and ages in shining robes of pure white linen.
These initiates were told to fast and prepare for initiation:
Meanwhile, like her other votaries, I should immediately abstain from unholy forbidden foods so that I might the better attain to the secret mysteries of this purest of religions.
The priest having put it like this, I did not allow my impatience to affect my obedience but, calmly and quietly and maintaining a commendable silence, I devoted myself in earnest to the sacred worship for some days.
When the day has come he is baptized:
Then, when the priest said the moment had come, he led me to the nearest baths, escorted by the faithful in a body, and there, after I had bathed in the usual way, having invoked the blessing of the gods he ceremoniously aspersed and purified me.
He then goes through more instruction, including secret teachings and continued fasting:
Next I was taken back to the temple, the day being now two-thirds over, where he made me stand at the goddess’s feet and privately gave me certain instructions which are too sacred to divulge. Then with everybody present he ordered me to abstain from the pleasures of the table for the next ten days and not to eat the flesh of any animal or drink any wine. This abstinence I observed with reverential restraint as instructed.
The initiation cumulates with another secret teaching:
Then the day came which was fixed for my pledged appearance before the goddess. Towards sunset there came flocking from all sides crowds of people, all bearing different gifts in my honour, according to the ancient practice of the mysteries. Then the uninitiated were all made to leave, I was dressed in a brand-new linen robe, and the priest took me by the hand and conducted me to the very innermost part of the sanctuary.
I dare say, attentive reader, that you are all agog to know what was then said and done. I should tell you if it were lawful to tell it; you should learn if it were lawful to hear it.
What is that secret teaching? We do not know for sure, but is about death, rebirth, heaven, and hell:
I came to the boundary of death and after treading Proserpine’s threshold I returned having traversed all the elements; at midnight I saw the sun shining with brilliant light; I approached the gods below and the gods above face to face and worshipped them in their actual presence. Now I have told you what, though you have heard it, you cannot know. So all that can without sin be revealed to the understanding of the uninitiated, that and no more I shall relate.
As explained before the initiation began:
For the keys of hell and the guarantee of salvation were in the hands of the goddess, and the initiation ceremony itself took the form of a kind of voluntary death and salvation through divine grace. Such as might be safely entrusted with the great secrets of our religion, when they had passed through life and stood on the threshold of darkness, these the power of the goddess was wont to select and when they had been as it were reborn return them to a new lifespan.
Ambrose and Augustine were practicing a baptism, not described in the Bible, but one resembling the mystery religions. We see in Paul a baptism that happens the very hour a new believer comes to salvation. Paul’s jailer in Phillippi asks what he can do to be saved:
Act 16:30 And brought them out, and said, Sirs, what must I do to be saved?
Act 16:31 And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
Act 16:32 And they spake unto him the word of the Lord, and to all that were in his house.
Act 16:33 And he took them the same hour of the night, and washed their stripes; and was baptized, he and all his, straightway.
After baptism, the jailer returns Paul to jail that very night after which Paul refuses to leave jail until the leaders of the city beg him to do so. In the gospels, Jesus and John baptize all that come to them without any initiate process. There is no special clothing. There is no fasting and reflecting. There is no secret teachings. It is a simple and straightforward rite.
The baptisms performed by Ambrose and Augustine are additional evidence that the Church, very early on, became heavily paganized.