Martin Luther was once approached by a working man who wanted to know how he could serve the Lord. Luther asked him, "What is your work now?" The man replied, "I'm a shoemaker."
Much to the cobbler's surprise, Luther replied, "Then make a good shoe and sell it at a fair price."
He didn't tell the man to make "Christian shoes." He didn't tell him to leave his shoes and become a monk.
As Christians, we can serve God in a variety of vocations. And we don't need to justify that work, whatever it is, in terms of its "spiritual" value or evangelistic usefulness. We simply exercise whatever our calling is with new God-glorifying motives, goals, and standards.
Outwardly there may be no discernible difference between a non-Christian's work and that of a Christian. A transformational approach to culture doesn't mean every human activity practiced by a Christian (designing computers, repairing cars, selling insurance, or driving a bus) must be obviously and externally different from the same activities practiced by non-Christians.
Rather, the difference is found in the motive, goal, and standard. John Frame writes, "The Christian seeks to change his tires to the glory of God and the non-Christian does not. But that's a difference that couldn't be captured in a photograph."
So, while Christians are to separate from the self-glorifying motives and God-ignoring goals of the world (our spiritual separation), we're not to separate from the peoples, places, and things in the world (a spatial separation). We're to be morally and spiritually distinct without being culturally segregated. In the famous words of Abraham Kuyper, "There is not one square inch in the entire domain of our human existence over which Christ, who is Sovereign over all, does ...