“There are certain substances that are, by themselves, salutary; but when they are mixed they are turned into poison. That is what happens when Law and Gospel are mingled… there arises a third substance, when Law and Gospel are confounded in a sermon. The new substance is entirely foreign to either original substance and causes both of them to lose their virtue… It is therefore a matter of utmost necessity that these two kinds of God’s Word be well and properly distinguished. Where this is not done, neither the Law nor the Gospel can be understood, and the consciences of men must perish with blindness and error.”
C.F.W. Walther “The Proper Distinction Between Law and Gospel”
PS: on a sidebar, Walther is obviously the best looking theologian in the history of Lutheranism…