31 Now these are the nations which Yahweh left, to prove Israel by them, even as many of Israel as had not known all the wars of Canaan; 2 only that the generations of the children of Israel might know, to teach them war, at the least such as before knew nothing of it: 3 namely, the five lords of the Philistines, and all the Canaanites, and the Sidonians, and the Hivites who lived on Mount Lebanon, from Mount Baal Hermon to the entrance of Hamath. 4 They were left, to prove Israel by them, to know whether they would listen to the commandments of Yahweh, which he commanded their fathers by Moses.
Matthew Henry's Commentary on Judges 3:1-4
Commentary on Judges 3:1-7
(Read Judges 3:1-7)
As the Israelites were a type of the church on earth, they were not to be idle and slothful. The Lord was pleased to try them by the remains of the devoted nations they spared. Temptations and trials detect the wickedness of the hearts of sinners; and strengthen he graces of believers in their daily conflict with Satan, sin, and this evil world. They must live in this world, but they are not of it, and are forbidden to conform to it. This marks the difference between the followers of Christ and mere professors. The friendship of the world is more fatal than its enmity; the latter can only kill the body, but the former murders many precious souls.