Charles Ellicott Commentary Joshua 23:12-13

"Else if ye do at all go back, and cleave unto the remnant of these nations, even these that remain among you, and make marriages with them, and go in unto them, and they to you; know for a certainty that Jehovah your God will no more drive these nations from out of your sight; but they shall be a snare and a trap unto you, and a scourge in your sides, and thorns in your eyes, until ye perish from off this good land which Jehovah your God hath given you." — Joshua 23:12-13 (ASV)

if you ... make marriages with them ... the Lord your God will no more drive out. —The common sense of this warning is manifest. The God of Israel cannot treat as His enemies those whom Israel has united with itself, unless He also makes war on Israel. It was a long time before Israel learned the lesson how to live in the world without being of the world. It was not learned until after the Babylonian captivity, and when learned, it soon developed into a Pharisaical exclusiveness, which produced the very opposite effect to that which the law was intended to have.