Heart-work is hard work indeed. To shuffle over religious duties with a loose anal heedless spirit, will cost no great pains; but to set thyself before the Lord, and tie up thy loose and vain thoughts to a constant and serious attendance upon him; this will cost thee something. To attain a facility and dexterity of language in prayer, and put thy meaning into apt and decent expressions, is easy; but to get thy heart broken for sin, while thou art confessing it; melted with free grace while thou art blessing God for it; to be really ashamed and humbled though the apprehensions of Gods infinite holiness, and to keep thy heart in this frame, not only in, but after duty, will surely cost thee some groans and pains of soul. To repress the outward acts of sin, and compose the external part of thy life in a laudable manner, is no great matter; even carnal persons, by the force of common principles, can do this: but to kill the root of corruption within, to set and keep up an holy government over thy thought, to have all things lie straight and orderly in the heart, this is not easy.
[Heart-work] is a constant work. The keeping of the heart is a work that is never done till life is ended. There is no time or condition in the life of a Christian which will suffer an intermission of this work. It is in keeping watch over our hearts, as it was in keeping up Moses’ hands while Israel and Amalek were fighting. No sooner do the hands of Moses grow heavy and sink down, than Amalek prevails. Intermitting the watch over their own hearts for but a few minutes, cost David and Peter many a sad day and night.
[Heart-work] is the most important business of a Christian’s life. Without this we are but formalists in religion: all our professions, gifts and duties signify nothing. ” My son, give me thine heart,” is God’s request. God is pleased to call that a gift which is indeed a debt; he will put this honor upon the creature, to receive it from him in the way of a gift; but if this be not given him, he regards not whatever else you bring to him. There is only so much of worth in what we do, as there is of heart in it. Concerning the hears, God seems to say, as Joseph of Benjamin, “If you bring not Benjamin with you, you shall not see my face.” Among the Heathen, when the beast was cut up for sacrifice, the first thing the priest looked upon was the heart; and if that was unsound and worthless the sacrifice was rejected. God rejects all duties (how glorious soever in other respects) which are offered him without the heart. He that performs duty without the heart, that is, heedlessly, is no more accepted with God than he that performs it with a double heart, that is, hypocritically.
John Flavel, On Keeping the Heart (Kindle Edition, location 133)