Numbers 20:12 - Coke's Commentary on the Holy Bible

Because ye believed me not— Various have been the inquiries as to what the fault of Moses and Aaron was for which they were excluded the promised land. Some imagine that their manner of speaking to the people, Hear, ye rebels, &c. was culpable; and that it is to this the Psalmist refers, when he says of Moses, that he spoke unadvisedly with his lips, Psalms 106:33. Others suppose that he expressed his diffidence and want of faith in God, not only by smiting the rock twice, but by smiting it at all, as God bade him only speak unto the rock, Numbers 20:8 and, indeed, we find the rod sometimes made use of only as a sign, stretched out in view of the people. See Exodus 9:23. His fault, however, seems rather to have consisted in smiting the rock twice, which argued a mind impatient of delay, provoked, and seemingly doubtful of God's answering his first sign: God had commanded Moses (say the authors of the Universal History) to call for the water by only speaking to the rock; but he, whether out of impatience at their loud murmurs, or out of diffidence of the promised supply, smote it twice with his rod, and used some indecent expressions, either to the rock or the people; for which he was excluded from setting his foot in the promised land: Aaron was likewise excluded in the same sentence, doubtless, as having shared in the fault. See Saurin's 62nd Dissert.