Jamieson, Fausset and Brown on Hebrews 12:18

18. For--The fact that we are not under the law, but under a higher, and that the last dispensation, the Gospel, with its glorious privileges, is the reason why especially the Hebrew Christians should "look diligently," &c. (He 12:15, 16).

are not come--Greek, "have not come near to." Alluding to De 4:11, "Ye came near and stood under the mountain; and the mountain burned with fire ... with darkness, clouds, and thick darkness." "In your coming near unto God, it has not been to," &c.

the mount--The oldest manuscripts and Vulgate omit "the mount." But still, "the mount" must be supplied from He 12:22.

that might be touched--palpable and material. Not that any save Moses was allowed to touch it (Ex 19:12, 13). The Hebrews drew near to the material Mount Sinai with material bodies; we, to the spiritual mount in the spirit. The "darkness" was that formed by the clouds hanging round the mount; the "tempest" accompanied the thunder.