Why would we not be prepared to handle a visible appearance of God like people saw in the Old Testament?
Because this present age of grace is characterized by God's silence compared with His Old Testament dealings, people today neither expect angels to appear nor expect a visible manifestation of God. If He suddenly appeared as He did then, it would overwhelm us; we are not prepared for that, and God does not expect us to be.
Les explains that Sir Robert Anderson made the point that when we enter the age of grace, compared to God's dealing in the Old Testament, it is as if God is silent. In that silence, we just don't expect angels to appear. Les illustrates this by saying that if on his way home at 11 o'clock at night he saw a bunch of angels on the highway, he doubts he could take it, and he doesn't think we could either. In the same way, if God suddenly appeared as He did in the Old Testament, it would crack us up; we are not prepared for that, and God doesn't expect us to be. So God is silent today compared to those days. The context also notes that God even appeared to old pagan Pharaoh and revealed that the lady was a man's wife, Abram's wife.