Take from Where in the World Is the Church?: A Christian View of Culture and Your Role in It by Michael Horton.
Martin Luther knew that understanding that the acceptance of the sinner before a holy God was the result of an “alien righteousness” would necessarily lead to revolutions in human relations. Released from the inward focus, the believer was free to embrace the world as a spiritual and godly activity, instead of separating from it with the misunderstanding that he was thereby separating from sinfulness. “For even in the monk’s cell,” Luther recalled, “I still had that rascal (his own sinful self) right in there with me.” When common laypeople discovered the Gospel, they were so revolutionized by it that they wanted to do everything they could to promote it. Far from leading to moral laxity, it inspired zeal where there had been apathy. In fact, a cobbler asked Luther what he should now do since he had embraced the Gospel. What should his calling now be? Just as for the hypothetical lawyer I mentioned at the beginning who wanted to serve the Lord, this was an obvious question for a medieval person who had been trained to think that a great spiritual experience required special devotion in terms of a sacred calling. The Reformers response was as surprising to the cobbler as it might be to some of us today: “Make a good shoe and sell it at a fair price.” When asked what he would do if he knew Christ were coming back tomorrow Luther replied, “I would plant a tree.” In other words, God is so pleased with our ordinary, faithful activity in this world that Luther no longer felt that he had to be found in prayer or in “spiritual” exercises when Christ returned in order to receive His blessing. (20)
Also, R. Scott Clark recommends this book on a Christ and Culture reading list here.