When Moses, someone who found 'favour' in the sight of God, asked to see God's glory, God told him that he could not show his face to Moses, 'for man shall not see me and live'. Instead, as God makes his glory pass by, he covers Moses in the cleft of a rock with his hand so he will not be destroyed.
Moses earnestly desired to see God in all his fullness, but was prevented my the frailty of being a mere human. When Jesus tells us that the 'pure in heart will see God', we should be incredibly surprised. Something that even Moses was not able to do, will one day be open to the people of the kingdom.