Corpus of Latin Ecclesiastical Writers, Vol. LXII: Ambrose's Exposition of Psalm 118

For on the eighth day there is solemn cleansing, either by the Law Refers to Leviticus 12:3 regarding circumcision., because the solemnity of circumcision was established to be fulfilled on the eighth day, or because the whole world was stained by our sins and polluted by those seven days. But when the day of the resurrection came, we were brought to life together with the Lord Jesus, and we were raised in the newness of life, 5 preferring the grace of washing. Deservedly, we offer the firstborn living creatures in the image of the firstborn Son of God, a spiritual sacrifice of chastity and simplicity acceptable to God, not on the fourth or fifth day, lest the sacrifice be unclean or incomplete, but on the eighth day, on which we have all been not only revived but also confirmed in the resurrection of Christ. 11 Whence, although there is immediate full cleansing in baptism, nevertheless, because the baptized person ought to know the reason for that very washing and sacrifice, he does not offer a sacrifice until he enters the eighth day, so that, informed by the knowledge of the heavenly sacraments, he may not offer his gift to the sacred altars like a crude victim, but as one capable of reason, when he has begun to be more instructed, lest the ignorance of the offerer defile the mystery of the oblation.

3. The title of the psalm is Alleluia, that is, "praise of God." For in these hymns God is truly praised, in which there is remission of sins. Finally, in the previous psalm, the passion of the Lord was set forth, which washed this world so that it might make the peoples worthy to praise God with an immaculate mouth. The very elements of the letters, like all Hebrew names, are not empty or devoid of reasonable interpretation, whose meanings we shall reveal in their proper places. The one hundred and eleventh psalm was arranged in Hebrew through these letters at the beginnings of the verses and is entirely moral; and the one hundred and tenth seems to me to be written in this way. Finally, they have twenty-two verses and individual sentences are explained in individual verses; whence they are also asserted to be described through meters.