These words are found near the beginning of J C Ryle's booklet on The world (which must be a chapter in Practical Religion). See here.
And now, before I go a single step further, let me warn every reader of this paper that he will never understand this subject unless he first understands what a true Christian is. If you are one of those unhappy people who think everybody is a Christian who goes to a place of worship, no matter how he lives, or what he believes, I fear you will care little about separation from the world. But if you read your Bible, and are serious about your soul, you will know that there are two classes of (those who call themselves) "Christians"- converted and unconverted. You will know that what the Jews were among the nations of the Old Testament, this the true Christian is meant to be under the New. You will understand what I mean when I say that true Christians are meant, in like manner, to be a "peculiar people" under the Gospel, and that there must be a difference between believers and unbelievers. To you, therefore, I make a special appeal this day. While many avoid the subject of separation from the world, and many absolutely hate it, and many are puzzled by it, give me your attention while I try to show you "the thing as it is."