From John 1:16 the "fullness" of God could not be known through any ordinary man. It must come through the One who possesses the nature of God. At John 1:18 the word "God" is stated first and is without the definite article.
This indicates first, that the emphasis is on the word "God," and second, the nature of God should be the object of our focus. The statement is not referring to the impossibility of a vision of God, i.e (a theophany), but rather to His qualities.
"Hath seen" (heoraken) is in the Greek perfect tense indicating a past action of seeing which is held in the mind so that it may be related to others. The utter inability of "no man" is stressed in opposition to God who revealed himself in the Only Begotten.
I happen to use the NASB and many of the ancient manuscripts read "only begotten God" indicating that Jesus is both God and only begotten or unique, one of a kind. This is backed up by John 3:16.
Now, going to John 14:9 we read Jesus saying to Philip, "Have I been so long with you, and yet you have not come to know Me, Philip? He who has seen Me has seen the Father; how do you say show us the Father?" (This does not mean that Jesus Christ is the person of God the Father and is verified by John 14:10.
"Do you not believe that I am in the Father, and the Father is in Me? The words that I say to you I do not speak on My own initiative, but the Father abiding in Me does His works."
So what is the point that Jesus is making? The Father has no separate manifestation from the Son. The Son is the only manifestation and REVELATION of the Father. What is known of the Father is revealed through the Son. to see the Son is to see the essence of the Father. (John 1:1, 18; 10:30; 12:45; Colossians 1:15; Hebrews 1:3.
So, when we get to the Old Testament and read verses like Genesis 17:1,2, "Now when Abraham was ninety-nine years old, the Lord appeared to Abram and said to him, "I am God Almighty; Walk before Me, and be blameless. Verse 2, And I will establish My covenant between Me and you, And I will multiply you exceedingly."
This is a physical appearance of God Almighty to Abraham. How do I know that it's physical? Genesis 17:22, "And when He/God finished talking with him/Abraham, God went up from Abraham."
At Genesis 18:1, Now the Lord appeared to him/Abraham by the oaks of Mamre, while he was sitting a the tent door in the heat of the day." What we have in this chapter is the Lord God appearing to Abraham along with two angels. Also from verse 9 the Lord has a discussion with Abraham about the destruction of Sodom and Gomorrah.
Notice at verse 18:33, "And as soon as He/God finished speaking to Abraham the Lord departed; and Abraham returned to his place. I suspect the Lord departed straight up North. At Genesis 19:1 the two angels make their way to Sodom.
Going back now to Genesis 16:7 we see the appearance of the angle of the Lord as the angel of the Lord. He says to Hagar, verse 8, "Hagar, Sarai's maid, where have you come from and where are you going? And she said, I am fleeing from the presence of my mistress Sarai."
Verse 9, Then the angel of the Lord said to her, Return to your mistress, and submit yourself to here authority." Verse 10, Moreover, the angel of the Lord said to her, "I will greatly multiply your descendants so that they shall be too many to count." At verse 11 the angel of the Lord tells her she is with child. At verse 12, the angel of the Lord said her son will be like a wild donkey of a man, His hand will be against everyone, and everyone's had will be against him; And he will live in the east of all his brothers." (who do you suppose these people are?)
Hagar says at verse 13, "The she called the name of the Lord who spoke to her, "Thou art a God who sees"; for she said, Have I even remained alive here after seeing Him?"
Here's a question that I feel needs to be addressed? Is the angel of the Lord who multiplied Hagar's descendents the same "being" at Genesis 17:1-2 that multiplied Abraham descendants?
I say yes because I am convinced the angel of the Lord (who can be physically seen) is the preincarnate Jesus Christ who as I said is the only physical manifestation and revelation of God the Father.
Just as I said that Jesus Christ is not God the Father from John 14:9 in the New Testament, the pre-incarnate Jesus Christ is not an actual angel like Michael or Gabriel in the Old Testament. In fact, the angel of the Lord never appears in the New Testament as the angel of the Lord, although he is mentioned.