Deuteronomy 20:4

The reader will notice that as the people came near the battle, the priest was to speak to them first, and the officers second. This order is very beautiful. The priest came forward to unfold to the people their high privileges; the officers came to remind them of their holy responsibilities. Such is the divine order here. Privilege comes first, and then responsibility.

Notice that the priest proposes not a single condition here. He states in the most absolute way the relationship, and the privilege that flows from it, belonging to the Israel of God. He does not say, "The Lord your God will be with you, if you do such and such." … Grace proposes no conditions, raises no barriers, makes no stipulations. Its language is, "The Lord your God is with you … He goes with you … to fight for you … to save you." When the Lord fights for His people they are sure of victory.

The divine assurance was threefold: the Lord goes with you, to fight for you, to save you. This presupposes their obedience. If they were obedient, they had nothing to fear. Victory was on their side. And we too need to fear nothing in our warfare with the wicked spirits (Eph. 6). The Lord is on our side, and if we resist the devil he will flee from us. But if Israel was disobedient and departed from the Lord, He fought against them and their enemies overcame them.

He assures them of the divine presence with them. The Lord God was with them. This surely was enough. It did not matter in the smallest degree how many, how powerful, or how formidable their enemies were. Before the Lord of hosts, the God of the armies of Israel, they would all prove to be like chaff before the whirlwind.

The governing principle here, as elsewhere, is confidence in the Lord. He had taken His people, brought them out of Egypt into relationship with Himself, and was now placing them in His own land. … It was not a question of soldiery or strategy, of force or skill or trickery, but of the Lord their God. … So faintheartedness would be intolerable. It could only come from one thing: the people were thinking not of the Lord their God, but of themselves or of their enemies. … What secures victory is the certainty that our God calls us to the fight, that it is His battle, not ours. Where that is so, we are as sure of the end as of the beginning.

Only imagine Joshua treating the Canaanites on the principles of Romans 12! It would have been as glaring an inconsistency as it would be for us to act on the principle of Deuteronomy 20. Why is this? Simply because in Joshua's day God was executing judgment in righteousness, whereas now He is dealing in unqualified grace.

We are not called to fight for eternal life; we have that as God's free gift before we begin. We are not called to fight for salvation; we are saved before we enter the conflict. … The object we fight for is to make good, maintain, and carry out in practice our heavenly position and character, in the middle of the scenes and circumstances of ordinary human life, day by day.