He draws this from a section in J.R.R. Tolkein's trilogy, The Lord of the Rings. In the scene Pippin, one of the hobbits, finds himself face to face with the Witch King, the commander of the forces of darkness.
All hope is lost as the demon king bears down on Pippin ready to devour him. But, just as Pippin stares death in the face he hears the call of distant horns! The cavalry, the Riders of Rohan have come to Pippin's aid... to his salvation.
In that moment Tolkein writes these words, "From that day on Pippin could no longer hear the sound of distant horns without breaking down into tears"
From that day on every time Pippin heard the sound of distant horns he was reminded of the day he was rescued and his greatest enemy defeated.
Keller's point is simply this:
What are the horns of your salvation?
Those places, people, books, memories, scriptures which bring you back to the cross and to your rescue?
Because finding your horns will help to eclipse those besetting sins of your present experience and bring you back to that hill where you salvation was won and your greatest enemy defeated.
