When Joshua attacked the city of Ai, he used standard military tactics of deception. He deceived them into believing he had a smaller force than he did. He got them to come out of the city and separate from it by fleeing as in terror. But an ambush was set, and the city was destroyed.
This same warfare tactic was used by God against Satan. Just when Satan believed he had achieved victory by killing the Son of God, that death brought about his ultimate defeat.
1 Corinthians 2:7-8 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: 8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.
Colossians 2:15 And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it.
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