Herr, deine Augen sehen... BWV 102: chorus (Sie haben ein härter Angesicht...)

The harsh musical interval of the tritone, the "Diabolus in Musica" (devil in music), was strictly controlled in much of early music. So wouldn't it be striking and bold to make a melody out of two of them? 

In this scary cantata opening, Bach does exactly this to set a terrifying fugue on the words "They have made their faces harder than a rock" to depict those who have gone astray from God. The music is appropriately harsh with tritones abounding, showing us that Bach's counterpoint can serve chaos just as well as order. These lawbreakers are lost, but not a lost cause; Bach shows in the closing chorale that they are in fact, just like us all, able to ask for forgiveness in the end.