There is great force, then, in the word “kiss”; for it indicates that we should embrace this Son with our whole heart and see or hear nothing else than Christ, and Him crucified. But whoever looks for something else in religion or seeks something higher will deceive himself and wander from the way of salvation. We should use our reason and wisdom for other things, for managing the household, doing our jobs, for buying and selling. But when it comes to the worship of God, you should deny all access to reason and cling to this Son alone. It is not bad per se to change clothes, like a monk, to fast, chastise the body, and the like. Truly we object to these practices only because people maintain the godless notion that these things are useful for obtaining the forgiveness of sins. As long as you reject this notion and sincerely kiss the Son, that is, if you put your trust only in His merits and grace, you will live eternally, even if you keep a monk’s hood. This, then, is the highest worship of God. All else is only the tail end. And yet the world neglects the most important thing and trusts in the hypocritical changing of externals.
Luther’s works, vol. 12: