And the Lord spake unto Moses and Aaron, Because ye believed me not, to sanctify me in the eyes of the children of Israel, therefore ye shall not bring this congregation into the land which I have given them. Numbers 20:12.
Some would regard... [Moses’] sin as one that should be lightly passed over; but God sees not as man sees. When within sight of the hills of Canaan, the Israelites murmured because the stream that had flowed wherever they encamped ceased. The cries of the people were directed against Moses and Aaron, whom they accused of bringing them into the wilderness to perish. The leaders went to the door of the tabernacle and fell on their faces. Again “the glory of the Lord appeared,” and Moses was directed, “Take the rod, and gather thou the assembly together, thou, and Aaron thy brother, and speak ye unto the rock before their eyes; and it shall give forth his water, and thou shalt bring forth to them water out of the rock” (Numbers 20:8).
The two brothers went on before the multitude, Moses with the rod of God in his hand. They were now aged men. Long had they borne with the rebellion and obstinacy of Israel; but now, at last, even the patience of Moses gave way. “Hear now, ye rebels,” he cried; “must we fetch you water out of this rock?” (Verse 10). And instead of speaking to the rock, as God had commanded him, he smote it twice with the rod.
The water gushed forth in abundance to satisfy the host. But a great wrong had been done. Moses had spoken from irritated feeling... “Shall we bring water?” he questioned, as if the Lord would not do what He promised. “Ye believed me not,” the Lord declared to the two brothers, “to sanctify me in the eyes of the children of Israel” (Verse 12).
More than this, Moses and Aaron had assumed power that belongs only to God. The necessity for divine interposition made the occasion one of great solemnity, and the leaders of Israel should have improved it to impress the people with reverence for God and to strengthen their faith in His power and goodness. When they angrily cried, “Must we fetch you water out of this rock?” they put themselves in God’s place, as if the power lay in themselves. By these words they greatly dishonored Christ, their invisible Leader. God, not man, should have been glorified. The Lord reproved these leaders, and declared that they should not enter the Promised Land. Before the Hebrew host He demonstrated that the sin of the leader was greater than the sin of those who were led.—Manuscript 169, October 12, 1903, “Words of Warning Against Present Dangers.”
Reference: E.G. White, "The Upward Look," p. 29