21 My brethren, wherefore do you cry out, wherefore do you exult, wherefore do you love, unless that a spark of this love is there? What do you desire? I ask you. Can it be seen with the eyes? Can it be touched? Is it some fairness which delights the eyes? Are not the martyrs vehemently beloved; and when we commemorate them do we not burn with love? What is it that we love in them, brethren? Limbs torn by wild beasts? What is more revolting if you ask the eyes of the flesh?
What more fair if you ask the eyes of the heart? How appears in your eyes a very fair young man who is a thief? How shocked are your eyes! Are the eyes of the flesh shocked? If you interrogate them, nothing is more shapely and better formed than that body; the symmetry of the limbs and the beauty of the color attract the eyes; and yet, when you hear that he is a thief, your mind recoils from the man. You behold on the other hand a bent old man, leaning upon a staff, scarcely moving himself, ploughed all over with wrinkles.
You hear that he is just: you love and embracest him. Such are the rewards promised to us, my brethren: love such, sigh after such a kingdom, desire such a country, if you wish to arrive at that with which our Lord came, that is, at grace and truth. But if you covet bodily rewards from God, you are still under the law, and therefore you shall not fulfill the law. For when you see those temporal things granted to those who offend God, your steps falter, and you say to yourself: Behold, I worship God, daily I run to church, my knees are worn with prayers, and yet I am constantly sick: there are men who commit murders, who are guilty of robberies, and yet they exult and have abundance; it is well with them.
Was it such things that you sought from God? Surely you belonged to grace. If, therefore, God gave to you grace, because He gave freely, love freely. Do not for the sake of reward love God; let Him be the reward. Let your soul say, “One thing have I desired of the Lord, that will I seek after; that I may dwell in the house of the Lord all the days of my life, that I may behold the beauty of the Lord.” Do not fear that your enjoyment will fail through satiety: such will be that enjoyment of beauty that it will ever be present to you, and you shall never be satisfied; indeed you shall be always satisfied, and yet never satisfied.
For if I shall say that you shall not be satisfied, it will mean famine; and if I shall say you shall be satisfied, I fear satiety: where neither satiety nor famine are, I know not what to say; but God has that which He can manifest to those who know not how to express it, yet believe that they shall receive.
Source: Tractates on the Gospel of John (New Advent)