The people of God have been LONGING for God's presence for thousands of years. They pray about it, sing about it and yearn about it all the time. They grieve that they are separated; that the kingdom is far or out of reach.
But then, in Matt 17, the Lord shows up "on accident" as far as Peter James, and John were concerned. Poof. The Father is here. Peter is still yapping and inattentive. Suddenly, they have the TIGER'S Tail. They caught it. They actually HAVE obtained what generations of humans have longed for.
Bam: Terror and fall on face. Why?
Because of the Lord's power. and secondly:
Because of my sin, and if that hits too close to home, because of humanity's sin. Woe is me. The PRESENCE OF GOD ILLUMINATES my sin and our sin. What am I going to do about my sin problem? That is what the disciples were wondering as their faces were in the dirt. Listen and wonder: