Genuine, spiritual mourning for sin is the work of the Spirit of God. Repentance is too rare a flower to grow in nature's garden. Pearls grow naturally in oysters, but penitence never shows up in sinners except when divine grace produces it in them. If you have one particle of real hatred for sin, God must have given it to you, for human nature's thorns never produced a single fig. "That which is born of the flesh is flesh."1
True repentance is tied directly to the Savior. When we repent of sin, we must have one eye upon sin and the other upon the cross; or it will be even better if we fix both our eyes on Christ and see our transgressions only in the light of His love.
True sorrow for sin is eminently practical. No man can say he hates sin if he lives in it. Repentance makes us see the evil of sin not merely as a theory but experimentally [experientially]--as a burn victim dreads fire. We will be as afraid of it as a man who has recently been robbed is afraid of the thief on the highway; and we will shun it--shun it in everything--not only in large matters, but in small things, as men avoid little vipers as well as great snakes. True mourning for sin will make us very careful with our tongue in case it should say a wrong word; we will be very watchful over our daily actions in case in anything we offend, and each night we will end the day with painful confessions of shortcomings, and each morning awaken with earnest prayers that God would today hold us up so that we may not sin against Him.
Sincere repentance is continual. Believers repent until their dying day. This is not something we do only once at the beginning of our Christian lives. Nor is it an intermittent exercise. Every other sorrow passes with time, but this dear sorrow grows as we grow, and it is such sweet bitterness that we thank God He permits us to enjoy and to suffer it until we enter our eternal rest.