γνῶθι σεαυτόν (Know Thyself)

Many men live in the dark to themselves all their days; whatever else they know, they know not themselves. They know their outward estates, how rich they are, and the condition of their bodies as to health and sickness they are careful to examine; but as to their inward man, and their principles as to God and eternity, they know little or nothing of themselves. Indeed, few labor to grow wise in this matter, few study themselves as they ought; on which yet the whole course of their obedience, and consequently of their eternal condition, does depend. This, therefore, is wisdom, if we have any design to please God, or to avoid that which is a provocation to the eyes of his glory.

[...] There is a constant enemy unto it in everyone's own heart; and what an enemy it is we shall afterward show, for this is our design: to discover him to the uttermost.  [...]

Awake, therefore, all of you in whose hearts is anything of the ways of God! Your enemy is not only upon you, as on Samson of old, but is in you also.  he is at work, by all ways of force and craft, as we shall see. Would you not dishonor God and his gospel; would you not scandalize the saints and ways of God; would you not wound your consciences and endanger your souls; would you not grieve the good and holy Spirit of God, the author of all your comforts; would you keep your garments undefiled, and escape the woeful temptations and pollutions of the days wherein we live; would you preserved from the number of the apostates in these latter days? Awake to the consideration of this cursed enemy, which is the spring of these and innumerable other evils, as also of the ruin of all the souls that perish in the world!

~Overcoming Sin and Temptation, by John Owen, Kelly M. Kapic, and Justin Taylor (Chapter 1 of "Indwelling Sin")