In the middle of that, he warns them. "You may not mix with these nations" (Josh 23:7). He warns them, "If you turn back and cling to the remant of these nations remaining among you ... know for certain that the Lord your God will no longer drive out these nations before you ..." (Josh 23:12-13). Joshua, before he died, strongly cautioned Israel to avoid making close relationships with people whose values and beliefs were radically different than their own. It wouldn't just be unwise; it would be devastating. It is noteworthy that the book of Judges follows immediately on Joshua, and Judges is a circular set of stories where the people immerse themselves in sinful relationships with the surrounding people, fall under God's judgment until they repent, and God rescues them again.
It's not like this is the only place such a warning occurs in Scripture. David wrote
Blessed is the man who walks not in the counsel of the wicked, nor stands in the way of sinners, nor sits in the seat of scoffers; but his delight is in the law of the LORD, and on His law he meditates day and night (Psa 1:1-2).Paul wrote "Do not be deceived: Bad company ruins good morals (1 Cor 15:33). He also penned the famous, "Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness? (2 Cor 6:14).
And yet we read that Jesus ate with sinners. He's famous for it. He was called "a friend of tax collectors and sinners" (Matt 11:19). How do we reconcile the two? Well, the answer is in Paul's phrase: "What fellowship has light with darkness?" And if you look back at Joshua's warning, it's the same. It wasn't that they weren't allowed an interaction with the nations. It was that they weren't allowed to mix. They weren't allowed to marry. They weren't allowed to partner with them. It wasn't supposed to be ... κοινωνία -- koinōnia. That's the word used there for "fellowship". Koinōnia is an interesting concept because it defies the American sense of "individualism" and embraces a unity, a divesting of self in favor of the group, a like-mindedness. It's like "two fellows in one ship". It isn't merely proximity or acquaintance; it is walking the same path in unison (1 John 1:3-6). It is shared purposes and shared values and shared beliefs and shared lives. And while Jesus fellowshiped in that sense with His disciples, He did not do the same with the tax collectors and sinners.
Why are there so many disaffected youth? You know, they're raised in the church and they're taught the right things and they're educated properly and still they leave the faith and the family and the truth. Why? Why are there so many dissatisfied Christians? These are the ones that have decided to be spiritual but not religious. They're not ditching all beliefs -- just biblical ones. Why is it that "the faith that was once for all delivered to the saints" (Jude 1:3) is no longer in favor1? There are, I'm sure, many reasons. There is Satan, the roaring lion (1 Peter 5:8). There are those who are "not of us" (1 John 2:19), the tares among the wheat (Matt 13:24-30), the wolves in sheep's clothing (Matt 7:15). And it is largely this tendency in our society to thoroughly and completely ignore this warning: Bad company corrupts good morals. It is our willingness to submerge ourselves in the world, to commune with rather than simply engage sinners. It is largely our blind acceptance and embrace of the world (1 John 2:15), welcoming full partnership with those around us and eventually internalizing their worldview.
Bad company corrupts good morals. Not the '70's rock band, Bad Company. The company that follows the counsel of the wicked, that stands with sinners, that sits with scoffers, that partners with unbelievers, with lawlessness, with darkness. "What partnership has righteousness with lawlessness? Or what fellowship has light with darkness?" Paul thought the questions were rhetorical, that the answers were obvious. In today's individualistic, Americanized Christianity, we don't seem to agree. But we're certainly reaping the results2.
1 To be fair, the faith that was once for all delivered to the saints has always been in disfavor ... which was why Jude appealed to believers to contend for it against those who "have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ" (Jude 1:4) ... you know, like the prevalent approach today.
2 e.g., Dan Haseltine, 'Jars Of Clay' Lead Singer, Tweets Support For Gay Marriage or folks like Rachel Held Evans.