“[Indwelling sin] works by negligence of private communion with God in prayer and meditation. I have showed before how indwelling sin puts forth its deceitfulness in diverting the soul from watchfulness in and unto these duties. Here, if it prevails, it will not fail to produce a habitual declension in the whole course of obedience. All neglect of private duties is principled by a weariness of God, as he complains, “You have not called upon me, you have been weary of me” (Isa. 43:22). Neglect of invocation proceeds from weariness; and where there is weariness, there will be withdrawing from that whereof we are weary. Now, God alone being the fountain and spring of spiritual life, if there be a weariness of him and withdrawing from him, it is impossible but that there will a decay in the life ensue. Indeed, what men are in these duties (I mean as to faith and love in them), that they are, and no more. Here lies the root of their obedience; and if this fails, all fruit will quickly fail. You may sometimes see a tree flourishing with leaves and fruit, goodly and pleasant. After a while the leaves begin to decay, the fruit to wither, the whole to droop. Search, and you shall find [that] the root, whereby it should draw in moisture and fatness19 from the earth to supply the body and branches with sap and juice for growth and fruit, has received a wound, is [in] some way perished and does not perform its duty, so that though the branches are flourishing a while with what they had received, their sustenance being intercepted they must decay. So it is here. These duties of private communion with God are the means of receiving supplies of spiritual strength from him—of sap and fatness from Christ, the vine and olive. While they do so, the conversation and course of obedience flourishes and is fruitful—all outward duties are cheerfully and regularly performed; but if there be a wound, a defect, a failing, in that which should first take in the spiritual radical moisture, that should be communicated unto the whole, the rest may for a season maintain their station and appearance, but after a while profession will wither, fruits will decay, and the whole be ready to die. Hence our Savior lets us know: what a man is in secret, in these private duties, that he is in the eyes of God, and no more (Matt. 6:6); and one reason among others is, because they have a more vigorous acting of unmixed grace than any other duties whatsoever. In all or most particular duties, besides the influence that they may have from carnal respects, which are many, and the ways of their insinuation subtle and imperceptible, there is an alloy of gifts, which sometimes even devours the pure gold of grace, which should be the chief and principal in them. In these there is immediate intercourse between God and that which is of himself in the soul. If once sin, by its deceits and treacheries, prevails to take off the soul from diligent attendance unto communion with God and constancy in these duties, it will not fail to effect a declining in the whole of a man’s obedience. It has made its entrance, and will assuredly make good its progress.”

(Excerpt from Indwelling Sin in the book Overcoming Sin and Temptation (PDF) by John Owen, pp. 385-386)