Writes theologian B. B. Warfield (Works, 7:114):

We are sinners and we know ourselves to be sinners, lost and helpless in ourselves. But we are saved sinners; and it is our salvation which gives the tone to our life, a tone of joy which swells in exact proportion to the sense we have of our ill-desert; for it is he to whom much is forgiven who loves much, and who, loving, rejoices much.

Adolf Harnack declares that this mood was brought into Christianity by Augustine. Before Augustine the characteristic frame of mind of Christians was the racking unrest of alternating hopes and fears. Augustine, the first of the Evangelicals, created a new piety of assured rest in God our Savior, and the psychological form of this new piety was, as Harnack phrases it, “solaced contrition,” — affliction, for sin, yes, the deepest and most poignant remorse for sin, but not unrelieved remorse, but appeased remorse.

There is no other joy on earth like that of appeased remorse: it is not only in heaven but on earth also that the joy over one sinner that repents surpasses that over ninety and nine just persons who need no repentance.