Albert Barnes Commentary Psalms 66:7

"He ruleth by his might for ever; His eyes observe the nations: Let not the rebellious exalt themselves. Selah" — Psalms 66:7 (ASV)

He rules by His power forever - literally, “Ruling by His power forever.” The idea is that He does this constantly, in each age and generation. He has never ceased to rule; He never will. His dominion extends from age to age and will stretch forward forever. The power which He demonstrated in delivering His people He retains now and will retain forever. In that unchanging power, His people may trust; that unchanging power, the wicked should fear.

His eyes behold the nations - All nations, all people. He sees all their conduct. They can conceal nothing from Him. They should, therefore, stand in awe. The wicked have much to fear from One who sees all that they do and who has power to crush and destroy them. Compare the notes at Psalms 11:4.

Let not the rebellious exalt themselves - that is, to be lifted up with pride or to feel secure. They cannot overcome an Almighty God; they cannot escape from His power. The word "rebellious" here refers to those who are impatient under the restraints of the law of God and who are disposed to cast off His authority. The admonition is one that may be addressed to all who rebel against God in this way, whether they are nations or individuals. They must all alike feel the vengeance of His arm and fall beneath His power.