Then I will remove my hand and you will see my back; but my face must not be seen." (Exodus 33:23, NIV)Context: Exodus 33:12-23
Why did God only let Moses see His back? We must admit much of this passage is a mystery to us. To some degree, we can understand our inability to see God's face because of His holiness. But, why allow Moses to see His back? What God's back looked like is a total mystery. It's not the kind of passage a moviemaker likes to dramatize. But, what was God telling Moses by showing His back.
In Romans 9:15, Paul quotes from this context, "I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion." (Exodus 33:19b, NIV) The context of Romans 9 is God's part in the lives of people; in particular, in the lives of the Jews who didn't follow Christ. This implies God was telling Moses something about His interaction with people.
"I will be with you. And this will be the sign to you that it is I who have sent you: When you have brought the people out of Egypt, you will worship God on this mountain." (Exodus 3:12, NIV)
It's difficult to have confidence in God when we don't know what today or tomorrow will bring. What strengthens our faith is when we can remember God's response when we relied on Him in the pass.