4 Some, indeed, studying the words so frequently used by the apostle, about our being dead with Christ and raised together with Him, and misunderstanding the sense in which they are used, have thought that the resurrection is already past, and that no other is to be hoped for at the end of time: “Of whom,” he says, “are Hymenæus and Philetus; who concerning the truth have erred, saying that the resurrection is past already; and overthrow the faith of some.” The same apostle who thus reproves and testifies against them, teaches nevertheless that we are risen with Christ. How is the apparent contradiction to be removed, unless he means that this is accomplished in us by faith and hope and love, according to the first-fruits of the Spirit? But because “hope which is seen is not hope,” and therefore “if we hope for that we see not, we do with patience wait for it,” it is beyond question that there remains, as still future, the redemption of the body, in longing for which we “groan within ourselves.” Hence also that saying, “Rejoicing in hope, patient in tribulation.”
5. This renewal, therefore, of our life is a kind of transition from death to life which is made first by faith, so that we rejoice in hope and are patient in tribulation, while still “our outward man perishes, but the inward man is renewed day by day.” It is because of this beginning of a new life, because of the new man which we are commanded to put on, putting off the old man, “purging out the old leaven, that we may be a new lump, because Christ our passover is sacrificed for us;” it is, I say, because of this newness of life in us, that the first of the months of the year has been appointed as the season of this solemnity. This very name is given to it, the month Abib, or beginning of months. Again, the resurrection of the Lord was upon the third day, because with it the third epoch of the world began. The first Epoch was before the Law, the second under the Law, the third under Grace, in which there is now the manifestation of the mystery, which was formerly hidden under dark prophetic sayings. This is accordingly signified also in the part of the month appointed for the celebration; for, since the number seven is usually employed in Scripture as a mystical number, indicating perfection of some kind, the day of the celebration of Easter is within the third week of the month, namely, between the fourteenth and the twenty-first day.
Source: Letters (New Advent)