Know therefore that the Lord thy God, he is God, the faithful God, which keepeth covenant and mercy with them that love him and keep his commandments to a thousand generations;
A little context is needed to truly understand this verse properly. His people had been led from Egypt and were taking on armies much greater than theirs in order to take their promised lands. They needed to be reminded that despite their human frailty that God was with them and would ensure their victory. To achieve this, they needed to be faithful to His commandments forever.