2 Be we then willingly ignorant of that which the Lord would not have us know: and let us inquire what this title, “of the eighth,” means. The day of judgment may indeed, even without any rash computation of years, be understood by the eighth, for that immediately after the end of this world, life eternal being attained, the souls of the righteous will not then be subject unto times: and, since all times have their revolution in a repetition of those seven days, that perhaps is called the eighth day, which will not have this variety.
There is another reason, which may be here not unreasonably accepted, why the judgment should be called the eighth, because it will take place after two generations, one relating to the body, the other to the soul. For from Adam unto Moses the human race lived of the body, that is, according to the flesh: which is called the outward and the old man, and to which the Old Testament was given, that it might prefigure the spiritual things to come by operations, albeit religious, yet carnal.
Through this entire season, when men lived according to the body, “death reigned,” as the Apostle says, “even over those that had not sinned.” Now it reigned “after the similitude of Adam's transgression,” as the same Apostle says; for it must be taken of the period up to Moses, up to which time the works of the law, that is, those sacraments of carnal observance, held even those bound, for the sake of a certain mystery, who were subject to the One God. But from the coming of the Lord, from whom there was a transition from the circumcision of the flesh to the circumcision of the heart, the call was made, that man should live according to the soul, that is, according to the inner man, who is also called the “new man” by reason of the new birth and the renewing of spiritual conversation.
Now it is plain that the number four has relation to the body, from the four well known elements of which it consists, and the four qualities of dry, humid, warm, cold. Hence too it is administered by four seasons, spring, summer, autumn, winter. All this is very well known. For of the number four relating to the body we have treated elsewhere somewhat subtly, but obscurely: which must be avoided in this discourse, which we would have accommodated to the unlearned. But that the number three has relation to the mind may be understood from this, that we are commanded to love God after a threefold manner, with the whole heart, with the whole soul, with the whole mind: of each of which severally we must treat, not in the Psalms, but in the Gospels: for the present, for proof of the relation of the number three to the mind, I think what has been said enough.
Those numbers then of the body which have relation to the old man and the Old Testament, being past and gone, the numbers too of the soul, which have relation to the new man and the New Testament, being past and gone, a septenary so to say being passed; because everything is done in time, four having been distributed to the body, three to the mind; the eighth will come, the day of judgment: which assigning to deserts their due, will transfer at once the saint, not to temporal works, but to eternal life; but will condemn the ungodly to eternal punishment.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)