Isaiah 40:18 - Albert Barnes' Notes on the Bible

To whom then will ye liken God? - Since he is so great, what can resemble him? What form can be made like him? The main idea here intended to be conveyed by the prophet evidently is, that God is great and glorious, and worthy of the confidence of his people. This idea he illustrates by a reference to the attempts which had been made to make a representation of him, and by showing how vain those efforts were. He therefore states the mode in which the images of idols were usually formed, and shows how absurd it was to suppose that they could be any real representation of the true God. It is possible that this was composed in the time of Manasseh, when idolatry prevailed to a great extent in Judah, and that the prophet intended in this manner incidentally to show the folly and absurdity of it.