From the website, Catholic Harbor of Faith and Morals…
by St. Thomas Aquinas
4th Day of Lent: Saturday: The Grain of Wheat
Unless the grain of wheat falling into the ground die, itself remaineth alone.–John xii. 24.
We use the grain of wheat in two ways, for bread and for seed. Here the word is to be taken in the second sense, grain of wheat meaning seed and not the matter out of which we make bread. For in this sense it never increases so as to bear fruit. When it is said that the grain must die, this does not mean that it loses its value as seed, but that it is changed into another kind of thing. So St. Paul (i Cor. xv. 36) says, That which then thou sowest is not quickened, except it die first.
The Word of God is a seed in the soul of man, in so far as it is a thing introduced into man’s soul, by words spoken and heard, in order to produce the fruit of good works, The seed is the Word of God (Luke viii. II). So also the Word of God garbed in flesh is a seed placed in the world, a seed from which great crops should grow, whence it is compared in St. Matthew’s Gospel (xiii. 31, 32) to a grain of mustard seed.
Our Lord therefore says to us, “I came as seed, something meant to bear fruit and therefore I say to you, Unless the grain of wheat falling into the ground die, itself remaineth alone” which is as much as to say, “Unless I die the fruit of the conversion of the Gentiles will not follow.” He compares Himself to a grain of wheat, because He came to nourish and to sustain the minds of men, and to nourish and sustain are precisely what wheaten bread does for men. In the Psalms it is written, That bread may strengthen man’s heart (Ps. ciii. 15), and in St. John, The bread that I will give is my flesh for the life of the world(John vi. 52).
2. But if it die it bringeth forth much fruit (John xii. 25). What is here explained is the usefulness of the Passion. It is as though the gospel said, Unless the grain fall into the earth through the humiliations of the Passion, no useful result will follow, for the grain itself remaineth alone. But if it shall die, done to death and slain by the Jews, it bringeth forth much fruit, for example:
(i) The remission of sin. This is the whole fruit, that the sin thereby should be taken away (Isaias xxvii. 9). And this is the fruit of the Passion of Christ as is declared by St. Peter, Christ died once for our sins, the just for the unjust that he might offer us to God (i Pet. iii. 18).
(ii) The conversion of the Gentiles to God. I have appointed you that you shall go forth and bring forth fruit and that your fruit should remain (John xv. 16). This fruit the Passion of Christ bore, if I be lifted tip from the earth, I will draw all things to myself (John xii. 32).
(iii) The fruit of Glory. The fruit of good labours is glorious (Wis. iii. 15). And this fruit also the Passion of Christ brought forth; We have therefore a confidence in the entering into the Holies by the blood of Christ: a new and living way which He hath dedicated for us through the veil, that is to say, His flesh (Hebr. x. 19).