Do you mortify; do you make it your daily work; be always at it whilst you live; cease not a day from this work; be killing sin or it will be killing you.

from The Mortification of Sin, John Owen

To abstain from sin for any reason is, so far, good; but yet you may abstain from sin from a motive which will lend no virtue to your abstinence. Some abstain from sin from fear of men, or from hope of gain: as the thief is honest when he sees the policeman, and the beggar becomes pious when a dole is to be had at church. One sin will often kill another sin, as the miser shuns profligacy because he is too mean to spend his money riotously. But to abstain from sin because you love God—ay, that is the thing.

Barbed Arrows from the Quiver of C. H. Spurgeon

Repentance means that you realize that you are a guilty, vile sinner in the presence of God, that you deserve the wrath and punishment of God, that you are hell-bound. It means that you begin to realize that this thing called sin is in you, that you long to get rid of it, and that you turn your back on it in every shape and form. You renounce the world whatever the cost, the world in its mind and outlook as well as its practice, and you deny yourself, and take up the cross and go after Christ. Your nearest and dearest, and the whole world, may call you a fool, or say you have religious mania. You may have to suffer financially, but it makes no difference. That is repentance.

from Studies in the Sermon on the Mount, Martyn Lloyd-Jones