Behold, the Lord GOD will come with might, With His arm ruling for Him. Behold, His reward is with Him And His recompense before Him. (Isaiah 40:10)

He is said to come “with a strong hand”, or with great power, which his work required; which was to fulfil the law, satisfy divine justice, atone for sin, grapple and conflict with innumerable enemies, undergo the death of the cross, bear the curse of the law, and the wrath of God, and all in order to obtain eternal redemption for his people; for this he came from heaven to earth, not by change of place, but by assumption of nature.

and his arm shall rule for him; or he shall have sufficient power of himself to do the work he comes about; his own arm or power wrought salvation for him and for his people;

behold, his reward is with him; to give to those that trust in him, that believe in him, embrace his Gospel, and act according to it.

and his work before him; the work of redemption and salvation, which he was called unto, sent to do, and which, being given him, he agreed to do, was very toilsome and laborious, yet he took great delight in it, and has finished it.

Commentary from John Gill’s Exposition of the Bible

Behold! literally means “Be sure to see . . . !,” i.e. “Don’t miss this! It is an observable, objective fact!”