Mystery hymn

It's hymn time! However, this time there are a few missing pieces.

Our sermon yesterday centered on grace, within the framework of Titus 3.

"But when the kindness and love of God our Savior appeared, he saved us, not because of righteous things we had done, but because of his mercy."-Titus 3:4,5b

To illustrate this concept, we heard the words of "a hymn."

Law and terrors do but harden,
all the while they work alone;
But a sense of blood-bought pardon
Soon dissolves a heart of stone

I can almost visualize that hardness melting away as the reality of God's forgiveness strikes us.

The source of the hymn, however, is unknown to me. The most common place it seems to be quoted is in Charles Spurgeon's sermons. I've seen it credited to Hart, and also to Toplady, who wrote "Rock of Ages."

But I haven't found the entire hymn, nor the melody associated with the portion above.