What did Augustine mean by "The OT is the NT concealed. The NT is the OT revealed."?

Great question with some great answers. You do have the quote turned around, but I think everyone knows what you meant. The actual quote is “The new [testament] is in the old concealed; the old [testament] is in the new revealed.”

St Augustine Retractions: Book 2 XV Chapter 27. There is much typology in the OT. Isaac was taken up Mount Moriah, carrying the fuel with which he would be sacrificed and at the last moment, the animal is sacrificed instead on God's command. Mount Moriah is later the site of Golgotha, where Christ is Crucified and God so loved the world he carried this out. Manna from heaven (bread of the angels) given to the Children of Israel to keep them alive. Then the type of Christ being born in Bethlehem (Town of Bread) and placed in a manger (a feeding trough for animals). Finally the Discourse of the Bread in John 6- Feeding of 5,000. There will always be enough bread to feed those who want it. Christ tells his apostles he is the bread of Life and in front of his apostles he tells them they must eat his body and drink his blood or they will have no life. Many disciples leave as they cannot stomach this -sounds like cannibalism. Christ questions Peter 'Are you going too?' Peter just says 'Lord where shall we go? You have the words of eternal Life'.Christ realises God has revealed this to Peter. Christ reveals the meaning of Isaiah 22 and the sacking of Shabnar for disobedience and Eliakim is now the new Prime Minister-whatever he allows, will be allowed, what he dismisses will be dismissed. He is given the KEYS to the community, as Christ gives his authority to Peter for the Church, whom he knows will be left on the world when he has ascended to the Father. There are many OT references.The Ark of the Covenant, for example, contained the Rod of Aaron, the Words of Law (the tablets of the Commandments) and some manna from the desert. Mirroring this, Mary is called the New Ark of the Covenant, Jesus in her womb is both priest and victim, He is the Word and also the Eucharist or Holy Communion, saving the repentant Children of God. Then there is the garden of Eden-(Loss of innocence), Garden of Geths*m*ne, (beginning of Salvation). There is also the fact that the Geburah or mother of the King in the Royal House of David, was always the queen-not the king's wife. Mary was, following this tradition-functioning as queen for Christ. (Tradition began with Solomon and Bathsheba).This is a huge subject, revealing many truths.

The old is in the new revealed.

Despite what some denominations believe, the New Testament didn't replace the Old Testament. Rather, the New Testament expanded on what was taught in the old.

The New Testament explains the spiritual meaning of the physical laws and commandments (e.g. "don't kill" also means "don't hate").

The New Testament explains that the JHVH of the Old Testament, the "I AM", was actually the being that was incarnated as Jesus.

Christianity reveals an entirely new meaning to the Old Testament (and without changing its original meaning).

The new is in the old concealed.

The New Testament presents God's plan for humanity, and shows how this plan was foreshadowed by Old Testament holidays. God's plan of salvation is compared to an agricultural harvest.

• Passover's sacrificial lamb symbolize Jesus's sacrifice.
• The Days of Unleavened Bread symbolize repentance and the rejection of sin in favour of the bread of life.
• Pentecost symbolizes God's gift of revealed knowledge, and the gift of his spirit to the first-fruits of the harvest of mankind.
• The Feast of Trumpets symbolizes Jesus's return to Earth to save mankind from itself.
• The Day of Atonement symbolizes the forgiveness of sin and the removal of Satan at the start of the Millennium.
• The Feast of Tabernacles symbolizes Jesus temporarily living on Earth during the Millennium and the "restoration of all things".
• The Last Great Day symbolizes the Judgement of all of mankind at the end of the Millennium, when the wheat is separated from the chaff, and the chaff burned to ashes.

The seven holy festivals are celebrated annually as a reminder of God's plan. All of Christianity was present in the Old Testament, but until Jesus explained it, its true meaning couldn't be understood.

The OT is the NT concealed.

This has similarities to Hebrews 10:1

For the law having a shadow of the good things to come, not the very image of the things; by the selfsame sacrifices which they offer continually every year, can never make the comers thereunto perfect

Also Colossians 2:16-17

Let no man therefore judge you in meat or in drink, or in respect of a festival day, or of the new moon, or of the sabbaths, Which are a shadow of things to come, but the body is of Christ

The NT is the OT revealed.

The gospels, especially the gospel according to St. Matthew are saturated with actions and words of Jesus and others as being fulfillments of various OT passages.

Jesus spoke to two disciples on the road to Emmaus in Luke:

25 He said to them, “How unwise and slow you are to believe in your hearts all that the prophets have spoken! 26 Didn’t the Messiah have to suffer these things and enter into His glory?” 27 Then beginning with Moses and all the Prophets, He interpreted for them the things concerning Himself in all the Scriptures.

1 Peter 1:1:

10 Concerning this salvation, the prophets who prophesied about the grace that would come to you searched and carefully investigated. 11 They inquired into what time or what circumstances[f] the Spirit of Christ within them was indicating when He testified in advance to the messianic sufferings[g] and the glories that would follow.[h] 12 It was revealed to them that they were not serving themselves but you. These things have now been announced to you through those who preached the gospel to you by the Holy Spirit sent from heaven. Angels desire to look into these things.

In Deuteronomy 18, Moses foretells a prophet (which Hebrews affirms is Jesus):

15 “The Lord your God will raise up for you a prophet like me from among your own brothers. You must listen to him.

Proverbs 30:

I am more stupid than any other man,[c] and I lack man’s ability to understand. 3 I have not gained wisdom, and I have no knowledge of the Holy One. 4 Who has gone up to heaven and come down? Who has gathered the wind in His hands? Who has bound up the waters in a cloak? Who has established all the ends of the earth? What is His name, and what is the name of His Son— if you know?