90 Moses killing the Egyptian in defending one of his brethren reminds us naturally of the destruction of the devil, our assailant in this land of strangers, by our defender the Lord Christ. And as Moses hid the dead body in the sand, even so the devil, though slain, remains concealed in those who are not firmly settled. The Lord, we know, builds the Church on a rock; and those who hear His word and do it, He compares to a wise man who builds his house upon a rock, and who does not yield or give way before temptation; and those who hear and do not, He compares to a foolish man who builds on the sand, and when his house is tried its ruin is great.
Source: Reply to Faustus the Manichaean (New Advent)