Tertullian

For Joshua was to introduce the people into the land of promise, not Moses. Now he called him an angel on account of the magnitude of the mighty deeds which he was to achieve (which mighty deeds Joshua the son of Nun did, as you can yourselves read) and on account of his office of prophet announcing the divine will. Similarly the Spirit, speaking in the person of the Father, calls the forerunner of Christ, John, a future angel, through the prophet: "Behold, I send my angel before your"—that is, Christ's—"face, who shall prepare your way before you."