Pope Shenouda III
We make the sign of the cross because Satan fears it
All Satan's efforts ever since Adam's creation and until the end of ages, came to naught on the Cross. God paid the price by His blood. He effaced with His blood the sins of all people who believe and obey Him. Therefore whenever Satan sees the cross, he trembles, remembering his great defeat and the loss of his strivings, is disgraced and retreats.
Thus the children of God use the sign of the cross because it is the sign of victory and the power of God. They are filled with power within, and the enemy trembles without.
The lifting up of the serpent in the past, which was a cure for people and salvation from death, resembles the lifting up of the Lord of glory on the Cross. It also resembles the sign of the cross in its efficacies (John.3: 14).