Why can't the world tell Christians apart from non-Christians?
The world cannot tell Christians apart because believers have lost the biblical concept of separation. Scripture calls them to come out from ungodly circumstances and be distinct, to be different while still living in the world, not participating in its ways, rather than blending in with it.
Les explains that the issue is the lost concept of separation, as shown in passages like 2 Corinthians 6:17 and 1 Corinthians 6. God's invitation is to come out from among an ungodly set of circumstances and be separated. Les teaches this is not a condition placed on salvation; under grace God never says "you will or else." Instead, it is a beseeching, a "yield not," still left up to the believer. The whole matter of separation has been lost sight of, even in some fundamental Bible-teaching and preaching churches. The result is that the world cannot tell a Christian when it sees one, and Christians cannot tell a Christian when they see one. Les says believers are to be different, but that does not mean putting one's nose in the air or having no dealings with the world. We are in the world, but not to be of it.