The statement about seeing God face-to-face has to do with God as He really is in the fullness of His glory. It does not rule out seeing God in one of His visible manifestations. In fact, people in the Bible did see God in His visible manifestations and survived. This was true with Hagar, for example, in Genesis 13:13. If you look at Genesis 32:30, you will notice what Jacob says: For I have seen God, face to face, and my life is preserved. Indeed, Jacob saw God face-to-face, but only in a visible manifestation of an angel and not God as he really is; otherwise, Jacob would be dead. In Isaiah 6:1, Isaiah claims to have seen God on His throne, as does Ezekiel in his vision. In the case of Jesus the Messiah, this is when God became Man, so in the form of man, one could look upon Him and not die as Jacob could look upon God in the form of an angel and not die.