The reason no one has seen God at any time, as John 1:18 affirms, is that God is invisible.
For the invisible things of him from the creation of the world are
clearly seen, being understood by the things that are made, even his
eternal power and Godhead; so that they are without excuse: (Romans
1:20, KJV)
This verse in Romans explains that God's "eternal power and Godhead" are "invisible things." If that were the only text on the matter, we might still find it unclear...but there are others.
Who is the image of the invisible God, the firstborn of every
creature: (Colossians 1:15, KJV)
Now unto the King eternal, immortal, invisible, the only wise God,
be honour and glory for ever and ever. Amen. (1 Timothy 1:17, KJV)
Clearly, these scriptures teach the invisibility of God. Interestingly, we don't often consider this aspect of God's nature, thinking more often of God's immortality, omnipotence, omniscience, and omnipresence. But we should add "invisibility" to the list.
In Hebrews we read:
By faith he forsook Egypt, not fearing the wrath of the king: for he
endured, as seeing him who is invisible. (Hebrews 11:27, KJV)
The expression "as seeing" in this text clearly denotes seeing by faith. That is, of course, what the chapter is all about, and it is often called "the Faith Chapter" or the Bible's "Hall of Faith."
As its first verse states:
Now faith is the substance of things hoped for, the evidence of things
not seen. (Hebrews 11:1, KJV)
But how did Moses "see" God?
And the LORD spake unto Moses face to face, as a man speaketh unto his
friend. And he turned again into the camp: but his servant Joshua, the
son of Nun, a young man, departed not out of the tabernacle. (Exodus
33:11, KJV)
Moses, with whom God said He would speak "face to face," was not actually permitted to see God's face. We read only a few verses later:
And I will take away mine hand, and thou shalt see my back parts: but
my face shall not be seen. (Exodus 33:23, KJV)
Clearly, one can speak face-to-face without seeing the face of the other individual. Consider the next case....
What about Jacob?
And Jacob called the name of the place Peniel: for I have seen God
face to face, and my life is preserved. (Genesis 32:30, KJV)
In this verse, Jacob claims to "have seen God face to face." But had he? The verses just prior to this give the important context needed to understand the situation.
And Jacob was left alone; and there wrestled a man with him until the
breaking of the day. (Genesis 32:24, KJV)
And when he saw that he prevailed not against him, he touched the
hollow of his thigh; and the hollow of Jacob's thigh was out of joint,
as he wrestled with him. (Genesis 32:25, KJV)
And he said, Let me go, for the day breaketh. And he said, I will not
let thee go, except thou bless me. (Genesis 32:26, KJV)
And he said unto him, What is thy name? And he said, Jacob. (Genesis
32:27, KJV)
And he said, Thy name shall be called no more Jacob, but Israel: for
as a prince hast thou power with God and with men, and hast prevailed.
(Genesis 32:28, KJV)
And Jacob asked him, and said, Tell me, I pray thee, thy name. And he
said, Wherefore is it that thou dost ask after my name? And he blessed
him there. (Genesis 32:29, KJV)
The story shows that Jacob mistook his "assailant" for an enemy, not knowing Who was with him--and he wrestled for his life. This was also done in the darkness of night when Jacob could not see who it was. Before daybreak, the heaven-sent One requested to leave, because dawn was soon to break. In other words, whomever it was did not wish to be seen. Furthermore, Jacob's night companion refused to identify himself.
How then had Jacob "seen" God? Only by faith.
What about all the Israelites?
And they will tell it to the inhabitants of this land: for they have
heard that thou LORD art among this people, that thou LORD art seen
face to face, and that thy cloud standeth over them, and that thou
goest before them, by day time in a pillar of a cloud, and in a pillar
of fire by night. (Numbers 14:14, KJV)
Did the Israelites see God face to face?
The text says the Canaanites had heard this, but the text does not actually assert it as an undeniable fact. But more than this, seeing by faith is certainly a part of this.
Remember, God's glory was always something that the people were expressly forbidden to look upon. God's Shekinah glory that resided above the mercy seat of the ark of the covenant, was not to be viewed by the people, and a covering was draped over the ark, concealing it, whenever it was moved by the priests. But when the Philistines returned the stolen ark, the record says:
And he [God] smote the men of Bethshemesh, because they had looked
into the ark of the LORD, even he smote of the people fifty thousand
and threescore and ten men: and the people lamented, because the LORD
had smitten many of the people with a great slaughter. (1 Samuel 6:19,
KJV)
Of course, these men of Bethshemesh had not actually seen God--but they had dared to look at this symbol of His presence, and were punished for even this.
There is no record of anyone having actually seen God; though there are texts in which people spoke as if they had. John, confronted by an angel, bowed in worship--perhaps thinking, mistakenly, that this angel was God Himself. We humans are easily confused when looking upon immortal beings. Yet the Bible is clear that no one has seen God at any time. That should settle the matter.