Jamieson, Fausset and Brown on Hebrews 6:8

8. that which--rather as Greek (no article), "But if it (the 'land,' He 6:7) bear"; not so favorable a word as "bringeth forth," He 6:7, said of the good soil.

briers--Greek, "thistles."

rejected--after having been tested; so the Greek implies. Reprobate ... rejected by the Lord.

nigh unto cursing--on the verge of being given up to its own barrenness by the just curse of God. This "nigh" softens the severity of the previous "It is impossible," &c. (He 6:4, 6). The ground is not yet actually cursed.

whose--"of which (land) the end is unto burning," namely, with the consuming fire of the last judgment; as the land of Sodom was given to "brimstone, salt, and burning" (De 29:23); so as to the ungodly (Mt 3:10, 12; 7:19; 13:30; Joh 15:6; 2Pe 3:10). Jerusalem, which had so resisted the grace of Christ, was then nigh unto cursing, and in a few years was burned. Compare Mt 22:7, "burned up their city" an earnest of a like fate to all wilful abusers of God's grace (He 10:26, 27).