The sincerity of our profession much depends upon the care we exercise in keeping our hearts. Most certainly, that man who is careless of the frame of his heart, is but a hypocrite in his profession, however eminent he be in the externals of religion. . . . It is true, there is great difference between Christians themselves in their diligence and dexterity about heart work; some are more conversant with, and more successful in it than others: but he that takes no heed to his heart, that is not careful to order it aright before God, is but a hypocrite. “And they come unto thee as the people cometh, and they sit before thee as my people, and they hear thy words, but they will not do them: for with their mouth they show much love, but their heart goeth after their covetous.” Here was a company of formal hypocrites, as is evident from that expression, as my people; like them, but not of them. And what made them so? Their outside was fair; here were reverent postures, high professions, much seeming delight in ordinances;” thou art to them as a lovely song:” yea; but for all that they kept not their hearts with God in those duties; their hearts were commanded by their lusts, they went after their covetousness. Had they kept their hearts with God, all had been well: but not regarding which way their hearts went in duty, there lay the essence of their hypocrisy.
If any upright soul should hence infer, ‘I am a hypocrite too, for many times my heart departs from God in duty; do what I can, yet I cannot hold it close with God: ‘I answer, the very objection carries in it its own solution. Thou sayest, ‘Do what I can, yet I cannot keep my heart with God.’ Soul, if thou doest what thou canst, thou hast the blessing of an upright, though God sees good to exercise thee under the affliction of a discomposed heart.
There still remains some wildness in the thoughts and fancies of the best to humble them; but if you find a care before to prevent them, and opposition against them when they come, and grief and sorrow afterward, you find enough to clear you from the charge of reigning hypocrisy. This precaution is seen partly in laying up the word in thy heart to prevent them. “Thy word have I hid in mine heart, that I might not sin against thee.” Partly in your endeavors to engage your heart to God; and partly in begging preventing grace from God in your commencement of duty. It is a good sign to exercise such precaution. And it is an evidence of uprightness, to oppose these sins in their first rise. “I hate vain thoughts.” “The spirit lusteth against the flesh.” Thy grief also discovers the uprightness of thy heart. If with Hezekiah thou art humbled for the evils of thy heart, thou hast no reason, from those disorders, to question the integrity of it; but to suffer sin to lodge quietly in the heart, to let thy heart habitually and without control wander from God, is a sad, a dangerous symptom indeed.
John Flavel, On Keeping the Heart (Kindle Edition, location 179)