John Gill Commentary Deuteronomy 32:9

"For Jehovah`s portion is his people; Jacob is the lot of his inheritance." — Deuteronomy 32:9 (ASV)

For the Lord's portion is his people, Jacob is the lot of
his inheritance .

This is the reason why the Lord so early provided a portion or inheritance for the children of Israel in the land of Canaan; because they were his part, his portion, his inheritance, which he chose by lot for himself, or allotted to himself; whom he chose to be his special and peculiar people; for though all the world is his, he only reserved a part for himself, which he separated from all the rest, and considers as his portion and inheritance, see (Psalms 33:12) ;

Thus the spiritual Israel of God, as they are his people, whom he has chosen, taken into covenant, given to Christ, and are redeemed and saved by him; they are his part or portion, separated by distinguishing grace from the rest of the world; and are the inheritance of Christ, who is appointed heir of all things, and is an unalienable inheritance; and is obtained by lot, or rather is measured out by a rod or line; by the line of electing grace, by which the church and people of God are circumscribed, marked out, and distinguished from others; and by the line and rule of the sacred Scriptures, which are the measure and standard of faith and practice, of worship and discipline to them.