The instructions were crystal clear. The Israelites were to completely drive the inhabitants of Canaan from the land. Not only that, they were to destroy every symbol and location designated for idol worship. There was an accompanying warning as well, "If you do not drive out the inhabitants of the land…those whom you let remain shall be irritants in your eyes and thorns in your sides, and they shall harass you in the land where you dwell, so says Numbers 33:50-55. Too, Deuteronomy 7:2-4 forbade making covenants and contracting marriages with the inhabitants Canaan because the hearts of their sons and daughters would be turned away from following God.
Unfortunately, the books of Joshua and Judges highlight several instances where the various Israelite tribes did not evict the existing residents (Joshua 13:13; 16:10; Judges 1:21, 27, 32). Apparently, it seemed easier and more advantageous (they became forced laborers - Judges 16:10) to allow those nations to stay. A consistent trait of the God of Heaven is that He will not force people into complete obedience; alternatively, He will allow people to experience the consequences of their disobedience.
Indeed, Israel's failure afforded God the opportunity to further test their faithfulness to Him. Judges 3:4 says, "and they were left (these evil nations JD), that He might test Israel by them, to know whether they would obey the commandments of the Lord, which He had commanded their fathers by the hand of Moses." How did they do on the test? They failed! Judges 3:5-6 says, "they took their daughters to be their wives, and gave their daughters to their sons; and they served their gods.” Tragically, the warning became the reality.
One might wonder, "Is there a parallel today." There is a very similar threat among churches of Christ. We have become content to dwell in a land filled with people whose hearts, while often religious, are not set on following the Word of the Lord. They have compromised on acceptable worship (John 4:24). They have substituted manmade schemes for the plan of salvation (Romans 6:17). They have modified the name and nomenclature that for centuries has identified the Spiritual Israel of God (Galatians 6:26) and distinguished her from the heathen, denominational religions (Romans 16:16).
Some surmise that since God allows such religious disparity to coexist in the world, that a little religious error must not really matter to Him. Have we not learned anything from Israelite history (Romans 15:4)? Perhaps, denominationalism is our test. Since we have stopped confronting error both privately and publicly, maybe God is content to let them dwell among us to see if we will hold true to His Word. What has been the result? We have churches of Christ compromising when it comes to acceptable worship. We have brethren who are no longer sure about the plan of salvation. We have congregations whitewashing their signs and billboards to remove the "church of Christ" designation from their property. Why? It's easier to dwell among the heathen than it is to stand boldly on the Word of God. Perhaps the drought in religious distinctiveness and true conversions has more to do with the failure of the Lord's church to drive out error with the Sword of the Spirit (Ephesians 6:17) than it has to do with disinterest among the people of the land. We will never change the world by becoming like the world.