"It were an easy thing to be a Christian,if religion stood only in a few outward works and duties,
But to take the soul to task, and to deal roundly with our own hearts, and to let conscience have its full work, and to bring the soul into spiritual subjection unto God,
This is not so easy a matter, because the soul out of self-love is loath to enter into itself, lest it should have other thoughts of itself than it would have."
[Richard Sibbes (1577-1635) was a Puritan pastor. Quotation is from The Soul's Conflict, and Victory Over Itself by Faith, XV:vii:6, in The Complete Works of Richard Sibbes, Vol. 1, (Edinburgh: James Nichol, 1862), p. 200.]
[UPDATE: I intended to but forgot to mention that I found this quote in David Powlison's book Speaking the Truth in Love.]