Augustine of Hippo

"The Lord is the portion of Mine inheritance, and of My cup" [Psalm 16:5]. For together with Me they shall possess the inheritance, the Lord Himself. Let others choose for themselves portions, earthly and temporal, to enjoy: the portion of the Saints is the Lord eternal. Let others drink of deadly pleasures, the portion of My cup is the Lord. In that I say, "Mine," I include the Church: for where the Head is, there is the body also. …

Exposition on Psalm 16 ↗

Jerome

The Lord is the portion of my inheritance and of my cup: it is you who will restore my inheritance to me. Our Savior in the Scriptures is called both high priest and king. High priest, in this: "You are a priest forever according to the order of Melchizedek." King, as in that place: "But I have been established as king by him over Zion, his holy mountain." According, then, to the fact that he is a priest, the Lord is his portion. …

Homily 61, on Psalm 15 ↗

Thomas Aquinas

"The Lord." Here he assigns the reason for his clinging to God alone, because he alone is his inheritance; as if to say, I cling to him alone because this is my inheritance. First he says that God is his inheritance. Second he commends it, saying that he is content with it, at "For indeed my inheritance," etc. …

Exposition on the Psalms of David ↗