19 “Man, his days are but as grass”. Let man consider what he is; let not man be proud. “His days are but as grass.” Why is the grass proud, that is now flourishing, and in a very short space dried up? Why is the grass proud that flourishes only for a brief season, until the sun be hot? It is then good for us that His mercy be upon us, and from grass make gold. “For he flourishes as a flower of the field.” The whole splendour of the human race; honour, powers, riches, pride, threats, is the flower of the grass.
That house flourishes, and that family is great, that family flourishes; and how many flourish, and how many years do they live! Many years to you, are but a short season unto God. God does not count, as you do. Compared with the length and long life of ages, all the flower of any house is as the flower of the field. All the beauty of the year hardly lasts for the year. Whatever there flourishes, whatever there is warmed with heat, whatever there is beautiful, lasts not; nay, it cannot exist for one whole year.
In how brief a season do flowers pass away, and these are the beauty of the herbs! This which is so very beautiful, this quickly falls. Inasmuch then as He knows as a father our forming, that we are but grass, and can only flourish for a time; He sent unto us His Word, and His Word, which abides for evermore, He has made a brother unto the grass which abides not. Wonder not that you shall be a sharer of His Eternity; He became Himself first a sharer of your grass. Will He who assumed from you what was lowly, deny unto you what is exalted in respect of you?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)