For the figure of the cup, see Psalm 11:6. It had already become a synonym for "condition in life."
Thou maintainest.--The Hebrew word is peculiar, and causes grammatical difficulties; but the sense is clear. God does not only dispose (cast) the lot of the man in covenant relation to Him--He does that even for unbelievers--but holds it fast in His hand. (See this use of the verb, Amos 1:5; Amos 1:8; Proverbs 5:5.) At the same time Hitzig's conjecture (tomid for tomikh), is very plausible, "Thou art ever my lot."
"the Lord is he who gives me the portion of mine inheritance;''
meaning his church and people, all the elect of God, who are Christ's portion and inheritance, given him by the Father; see Deuteronomy 32:9; And assigns to me my cup, as of blessings, so of sorrows and sufferings, which being measured out, filled up, and put into his hand by his Father, he freely took it, John 18:11;
thou maintainest my lot; that is, either his interest in God himself, as his covenant God, which always continued; or the lot of goods, of grace and glory, put into his hands for his people, which always remains; or rather the saints themselves, who, as they are Christ's portion and inheritance, so they are his lot; in allusion to the land of Canaan, which was divided by lot: these Jehovah took hold of, kept, preserved, and upheld, as the word (s) signifies; so that they shall never totally and finally fall and perish; and this sense is countenanced by what follows.
(s) "sustentas", Musculus, Pagninus, Junius & Tremellius, Piscator; so Ainsworth; "sustentans", Montanus, Michaelis; "tenuisti", Cocceius; "tenendo quasi sustentans", Gejerus.