Daily Luther: How Believers Live in the World
Faith can use all things for its purpose, whether good or bad, except unbelief and its fruits. For these are directly contrary to faith and do not permit faith to remain; those things that do permit faith to remain are themselves rendered harmless by faith, are made pure, holy, useful, and salutary, so that the believer may live with them and keep them without danger. If this were not so, no Christian could live, for he is forced to live among evil and non-Christian people. But if he does not follow them but puts them to good use, he may live with or among them to the end that they may gain piety and become Christians.
To a Christian, therefore, the entire world is holiness, purity, utility, and piety. Contrariwise, to a non-Christian the whole world is unholiness, impurity, uselessness, and destruction—even God with all His goodness, as Ps. 18:26–27 says to God: “With the pure Thou dost show Thyself pure; and with the crooked Thou dost show Thyself perverse.” Why is this? Because the pure, that is, the believers, can use all things in a holy and blessed way to sanctify and purify themselves. But the unholy and the unbelievers sin, profane, and pollute themselves incessantly in all things. For they cannot use anything in a right, godly, and blessed way, so that it might serve their own salvation.