The people of Israel may not pride themselves—no boasting. They received the land and did not conquer it in their own strength. It happened through God’s powerful intervention. The light of God’s face expresses that God loves his people. Someone who prays in need is often seeking God’s face. Let God look upon me pleasingly.
Think of the blessing of Numbers 6:25: “the Lord make his face to shine upon you and be gracious to you.” In Psalm 147:11 we see that God does not like power houses “but the Lord takes pleasure in those who fear him, in those who hope in his steadfast love.” A second explanation is also possible: the enemy retreats before God’s face on which his wrath can be seen. It is less likely, considering the connection of the light of your face, for you delighted in them.
3 for not by their own sword did they win the land, nor did their own arm save them, but your right hand and your arm, and the light of your face, for you delighted in them.