Isaiah 55:4

Behold, I have given him for a witness to the people
That is, the Messiah, as Aben Ezra, Kimchi, and Ben Melech rightly interpret it. This respects an act past in eternity, in God's eternal purposes and decrees, when he appointed Christ to the office of a Mediator; and this was an act of his grace, a free gift of his, flowing from his love to his people, both Jews and Gentiles, even all his elect, to whom Christ is a "witness", both of his father and of himself: of his father, of his good will to men, in forming the scheme of their salvation; of his love to sinners, in the mission of him; of his justice and holiness, which appear in his being the propitiation for sin; of his truth in his promises; of his whole mind and will, with respect to doctrine and worship: he is a witness of himself; of his deity and perfections; of his divine and eternal sonship; of his existence before his incarnation; of his Messiahship; of the end of his coming into the world; of his sufferings, death, and resurrection; of his second coming; and of the several characters he bears: he is a witness of the covenant itself, as well as the surety, Mediator, and messenger of it, and of truth in general; to which he has bore witness by his word and doctrines; by his works and miracles; by his sufferings and death; by the Scriptures of truth; by his Gospel, and the ministers of it; and by his spirit, and a faithful witness he is: a leader and commander to the people;
he is a "leader", as he is a teacher of his people, who teaches them to profit, and leads them in the way they should go; as a king that guides his subjects with the skilfulness of his hands, as David the type of him did; as a general leads out and on his armies to battle; as a shepherd leads his flock to good pastures; as a guide to those that know not the way; and as one that goes before others by way of example: Christ leads his people out of their own ways into his ways; and he leads them in a right way to the city of their habitation, to heaven at last; and he leads them on gradually and gently, as they are able to bear. He is a "commander" in a military way, a wise, powerful, valiant, and courageous one, and always victorious; and in a political sense, as a King commands his subjects, whose commands are to be obeyed; and indeed they are written on the hearts of his people; they are not grievous, though they cannot be performed in their own strength; nor is it designed that life and salvation should be obtained by the observance of them, but are done to testify subjection to Christ, and gratitude to him. The Targum is,

``behold, I have appointed him a Prince to the people, a King, and a ruler over all kingdoms.''