the minister must be "properly called", in the sense of the divine credentials essential to the office of ambassador of Christ. By saying "properly called" he wants to differentiate between the one who is truly called by God and set apart by Him to take charge of the "bride" of the Lamb, and another who has no such call or understanding of what the Church is or what it means to serve the Lord. You want to be a pastor for your own interest, for the prestige of having such a charge, or for personal benefits that such a charge entails. He appoints himself shepherd, but does not have those special credentials of Ambassador of God, because He has not called or named them.
To serve the church of Jesus Christ, he must have the divine support -as an ambassador- because, as the passage from Isaiah indicates, God wishes to give His Church consecrated and capable ministers as a fruit of the great love He has for her. . His purpose is to pour great blessing and spiritual glory on that particular group of his universal Church. Since His desire is that the "people" of the world and the "kings" of the earth see the "justice" and "glory" of God through His church, and that it be "a crown of glory in the hand of the Lord, and a diadem of kingdom in the hand of your God" (vss.2-3), the ambassador is to be chosen and empowered by the same Lord of the Church Otherwise, there will be neither the blessing nor the spiritual glory that God desires to give to the church.