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God is Spirit. His essential nature is spirit (“pneuma” John 4:24). Therefore, he cannot be seen by our eyes (John 1:18). Any language in the Bible that refers to someone seeing him (e.g., John 14:7 eorakate “seen, beheld”) is metaphorical or the language of accomodation (cf. 1 John 3:2-3). This, of course, does not mean that he cannot be known. In fact, it is precisely because he is an omnipresent, omniscient, personal being that he can be known by us in such a profound way. Our apprehension of his presence is more immediate than objects to our consciousness. Our knowing of God through His Spirit in us is a more intimate knowledge than we have even of our spouse (cf. Rom 5:1-5).
When God appears as an angel (malak) or as the preincarnate Christ (in the Garden of Eden Gen 18 and the three visitors Joshua 5:13-15) one still does not physically see God in his essence. One sees the representation of God in the form he took. Now when we come to the incarnation (John 1:1, 14) we have to be very careful. The writer of Hebrews (1:3) says “the son is the radiance of God’s glory and the exact representation of his being,” an idea which is similar to John 1:18. None of this means, however, that any man ever saw God in his essence. In John 1:18 the apostle John explicitly says that no one has ever seen God, but that Christ has interpreted or made him known. Once again, just because we cannot see him does not mean we cannot know him. This was as true for the people who knew Jesus personally during his time on earth as it is for us now.