I was taught that just as Eve was born from the body of Adam while he slept, so to was the Church born from the Body of Christ while He slept the sleep of death on the cross. Think of who was at the foot of the cross.
The reference in The Mystical City of God is a little obscure but it seems to place the baptism of the Virgin Mary early in Jesus Christ’s public ministry, after the call of his first disciples.
Then again, according to Catholic Theology, Mary was cleansed of the stain of original sin at her conception, so she didn’t need to be baptised anyway.
Then it wouldn’t be the Baptism into the Covenant. It would be John’s baptism of Repentance, that Mary would do "to fulfill all righteousness.
The reference in The Mystical City of God is not explicit enough to say what sort of baptism the Virgin Mary received. However, since it appears to be placed after the call of the first disciples, it was probably not John’s baptism of repentance but the baptism that Jesus and/or his disciples administered early in Jesus’ public ministry, as mentioned in John 3:22-27; 4:1-2. The Bible itself does not give enough info to say whether they administered a true Christian baptism at that time or something like John’s baptism of repentance. I could be wrong but my guess is that it was a true Christian baptism in a manner similar to the way the Last Supper was a true Eucharist, though it too occurred before the death and resurrection of Jesus Christ.
I wouldn’t place any of the baptisms in John as Sacramental. In John’s Gospel (During His Farewell speech at the Last Supper), Jesus is very clear in stating that He must die in order for the Holy Spirit to come. In chapter 4, John shows that Jesus did not baptize.
This is like the question, “Which came first, the Chicken or the Egg”? Mary gave birth to the Church, but wasn’t part of the Church, until she joined the Church?
Now we’re on the same page! My point, I guess, is that before we can go forward with the question, we may wish to go backward with it in addressing: When did the Church begin? If you answer that question with, “Never! It has always existed as the mystical body of Christ” then the thread will head in one direction, while if you answer with, "At Pentecost or at the Cross, the thread will go down another path. I doubt the OP or his protestant friend actually undestood the intricacies of the question…
The Church was conceived at the Cross. The Church was born at Pentecost. But if you still look at the effects of Baptism, we can see that Mary had all of these effects at the moment of Conception. This is all my humble opinion, so I agree that one guess is as good as another.
While we’re at it, why don’t we nail down these angels dancing on the head of a pin. To me it all depends, are they river-dancing, or are they square-dancing. Big difference, mind you!