Now I learn, thanks to Google, that the slogan is popular on merchandise aimed at the young Christian market, partially because of the 1999 song "In Not Of" by the Christian group Avalon -- since covered by Clay Aiken, of all people:
We are not like the world but we can love itI wonder whether anyone today wears this shirt in the spirit with which William Hazlitt would have worn it. His 1821 essay "On Living to One's-Self" reads, in part (emphasis mine):
Come bring the hope to hopeless men
Until the lost are found in Him
He came to save the world
So let us be in it, not of it
What I mean by living to one's-self is living in the world, as in it, not of it: it is as if no one knew there was such a person, and you wished no one to know it: it is to be a silent spectator of the mighty scene of things, not an object of attention or curiosity in it; to take a thoughtful, anxious interest in what is passing in the world, but not to feel the slightest inclination to make or meddle with it. It is such a life as a pure spirit might be supposed to lead, and such an interest as it might take in the affairs of men: calm, contemplative, passive, distant, touched with pity for their sorrows, smiling at their follies without bitterness, sharing their affections, but not troubled by their passions, not seeking their notice, nor once dreamt of by them.For more on this radical son of a Unitarian minister, visit the Hazlitt Society website.