There must have been somewhat particularly heinous in this sin. It is called reproaching the LORD. The HOLY GHOST hath given the best comment upon it by the mouth of his servant David, when he prayed to be kept from presumptuous sin, so that he might be preserved from the great transgression. Psalms 19:13. Perhaps it was similar to the unpardonable sin of blasphemy against the HOLY GHOST. Be it what it may, it stands herein this place as an awful monument in the midst of mercy, for which there remaineth no sacrifice, but a certain fearful looking for of judgment and fiery indignation. Hebrews 10:26-27.
Related Commentaries of Numbers 15:30
Numbers 15:30-31
30 But the soul that doeth ought presumptuously,c whether he be born in the land, or a stranger, the same reproacheth the LORD; and that soul shall be cut off from among his people.
31 Because he hath despised the word of the LORD, and hath broken his commandment, that soul shall utterly be cut off; his iniquity shall be upon him.