I classify this verse under strength, but it is best seen as how in our emotions and good intentions we try with our own strength of efforts to do God’s will….but we do it contrary to his command.
Num 20:7 And the LORD spake unto Moses, saying,
Num 20:8 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: so thou shalt give the congregation and their beasts drink.
Num 20:9 And Moses took the rod from before the LORD, as he commanded him.
Num 20:10 And Moses and Aaron gathered the congregation together before the rock, and he said unto them, Hear now, ye rebels; must we fetch you water out of this rock?
Num 20:11 And Moses lifted up his hand, and with his rod he smote the rock twice: and the water came out abundantly, and the congregation drank, and their beasts also.
Note this simple change from the command to the action….and it’s consequence.
Num 20:12 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.
Num 20:24 Aaron shall be gathered unto his people: for he shall not enter into the land which I have given unto the children of Israel, because ye rebelled against my word at the water of Meribah.
One use of this is illustrated here.
Amo 6:13 Ye which rejoice in a thing of nought, which say, Have we not taken to us horns by our own strength?
John Gill writes…And this also is the language of such persons, who ascribe regeneration and conversion, faith, repentance, the cleansing of a man’s heart, and the reformation of his life, yea, his whole salvation, to the power and strength of his free will, when man has no strength at all to effect any of these things; these are all vain boasts, and very disagreeable and offensive to the Lord; and for such like things persons stand here reproved by him, and threatened with woes; for woe must be here supplied from Amo 6:1.