24 “My heart and my flesh has failed, O God of my heart”. This then for me in Heaven has been reserved, “God of my heart, and my portion is my God.” What is it, brethren? Let us find out our riches, let mankind choose their parts. Let us see men torn with diversity of desires: let some choose war-service, some advocacy, some various and sundry offices of teaching, some merchandise, some farming, let them take their portions in human affairs: let the people of God cry, “my portion is my God.”
Not for a time “my portion;” but “my portion is my God for everlasting.” Even if I always have gold, what have I? Even if I did not always have God, how great a good should I have? To this is added, that He promises Himself to me, and He promises that I shall have this for everlasting. So great a thing I have, and never have it not. Great felicity: “my portion is God!” How long? “For everlasting.” For behold and see after what sort He has loved him; He has made his heart chaste: “God of my heart, and my portion is God for everlasting.”
His heart has become chaste, for nought now God is loved, from Him is not sought any other reward. He that does seek any other reward from God, and therefore is willing to serve God, more precious does make that which he wills to receive, than Him from whom he wills to receive. What then, is there no reward belonging to God? None except Himself. The reward belonging to God, is God Himself. This he loves, this he esteems; if any other thing he shall have loved, the love will not be chaste.
You are receding from the Fire immortal, you will grow cold, will be corrupted. Do not recede. Recede not, it will be your corruption, it will be your fornication. Now he is returning, now he is repenting, now he is choosing repentance, now he is saying, “my portion is God.” And after what sort is he delighted with that Same, whom he has chosen for his portion.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)