John Gill Commentary Deuteronomy 7:21

"Thou shalt not be affrighted at them; for Jehovah thy God is in the midst of thee, a great God and a terrible." — Deuteronomy 7:21 (ASV)

You shall not be affrighted at them
At their numbers, nor at their gigantic stature:

for the Lord your God is among you :
in the tabernacle, in the holy of holies, which was in the midst of them, and besides would give proof of his powerful presence among them, in protecting them, and destroying their enemies;

a mighty God and terrible ;
mighty to save his people, and terrible to others.