The passage that deals with Moses seeing the glory of the Lord is Exodus 33:18-23. This very passage answers the question you have. Let's look at it.
Exodus 33:18-23 And he said, I beseech thee, shew me thy glory. And he said, I will make all my goodness pass before thee, and I will proclaim the name of the LORD before thee; and will be gracious to whom I will be gracious, and will shew mercy on whom I will shew mercy. And he said, Thou canst not see my face: for there shall no man see me, and live. And the LORD said, Behold, there is a place by me, and thou shalt stand upon a rock: And it shall come to pass, while my glory passeth by, that I will put thee in a clift of the rock, and will cover thee with my hand while I pass by: And I will take away mine hand, and thou shalt see my back parts: but my face shall not be seen.
You will notice that Exodus 33:20 states, "Thou canst not see my face: for there shall no man see me, and live." Read this carefully and you will understand that to see the Lord is to see His face. In other words, no man can look on the fullness of the glory of God (His face) and live.
However, in several places in the Bible different people were able to see God while His glory was veiled in some way. When He appeared on Mount Sinai, He was veiled in a thick cloud. When He appeared to Moses, He allowed Moses to look only on his back parts. Men might see some sort of appearance of God, but he cannot look upon God in the fullness of His glory. When we understand the different ways of seeing God, we can understand John 1:18, which states, "No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him."
