Is there any place in the Old Testament that clearly teaches that the Messiah is God incarnate?
Yes! There are many places in Tanakh (the Hebrew Scriptures) that establish the fact that the Messiah must be no one less than God Himself who has come to us in the flesh. As the Son of Man, He is humankind’s representative before a holy and righteous God. As the Son of God, He’s God’s representative to a fallen human race. Only as a sinless man can the Messiah offer Himself as our perfect sacrifice. Only as Incarnate God can He forgive our sins and grant us eternal life. Perhaps one of the clearest places in the Hebrew Scriptures that teaches both the humanity and the deity of the Messiah is found in Isaiah 9:6 (in the Hebrew Bible, Isaiah 9:5). For unto us a child is born, unto us a son is given: And the government will be upon His shoulder. And His name shall be called Wonderful, Counselor, Mighty God, Everlasting Father, Prince of Peace.” This verse tells us very clearly that the Messiah will certainly be human (a child and a son). But at the same time, He is given titles that can only be ascribed to God. Perhaps the strongest title given to Him in this verse is “Mighty God.” In Hebrew, the words are “El Gibbor,” and this is the very same title given to God in Isaiah 10:21, where we read, “The remnant will return, the remnant of Jacob, to the Mighty God.” This is the miracle of Christmas, and it’s so wonderful that it’s almost beyond our ability to comprehend: the child who was born in a stable in Bethlehem was and is “Immanuel”—God with us!