31 Now these are the nations which the Lord kept in the land for the purpose of testing Israel by them, all those who had had no experience of all the wars of Canaan; 2 Only because of the generations of the children of Israel, for the purpose of teaching them war—only those who up till then had no experience of it; 3 The five chiefs of the Philistines, and all the Canaanites and the Zidonians and the Hivites living in Mount Lebanon, from the mountain Baal-hermon as far as Hamath: 4 For the purpose of testing Israel by them, to see if they would give ear to the orders of the Lord, which he had given to their fathers by the hand of 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.