2 Augustine replied: Let all who have given their hearts to Christ say whether they can listen patiently to these things, unless Christ Himself enable them. Faustus, full of the new honey, rejects the old vinegar; and Paul, full of the old vinegar, has poured out half that the new honey may be poured in, not to be kept, but to be corrupted. When the apostle calls himself a servant of Jesus Christ, called to be an apostle, separated unto the gospel of God, this is the new honey.
But when he adds, "which He promised before by His prophets in the Holy Scriptures of His Son, who was made of the seed of David according to the flesh," this is the old vinegar. Who could bear to hear this, unless the apostle himself consoled us by saying: "There must be heresies, that they which are approved may be made manifest among you?" Why should we repeat what we said already? — that the new cloth and the old garment, the new wine and the old bottles, mean not two Testaments, but two lives and two hopes,— that the relation of the two Testaments is figuratively described by the Lord when He says: "Therefore every scribe instructed in the kingdom of God is like an householder bringing out of his treasure things new and old." The reader may remember this as said before, or he may find it on looking back.
For if any one tries to serve God with two hopes, one of earthly felicity, and the other of the kingdom of heaven, the two hopes cannot agree; and when the latter is shaken by some affliction, the former will be lost too. Thus it is said, No man can serve two masters; which Christ explains thus: "You cannot serve God and Mammon." But to those who rightly understand it, the Old Testament is a prophecy of the New. Even in that ancient people, the holy patriarchs and prophets, who understood the part they performed, or which they were instrumental in performing, had this hope of eternal life in the New Testament.
They belonged to the New Testament, because they understood and loved it, though revealed only in figure. Those belonging to the Old Testament were the people who cared for nothing else but the temporal promises, without understanding them as significant of eternal things. But all this has already been more than enough insisted on.
Source: Reply to Faustus the Manichaean (New Advent)