Namely, that in spiritual and divine things the intellect, heart, and will of the unregenerate man are utterly unable, by their own natural powers, to understand, believe, accept, think, will, begin, effect, do, work, or concur in working anything, but they are entirely dead to what is good, and corrupt, so that in man's nature since the Fall, before regeneration, there is not the least spark of spiritual power remaining, nor present, by which, of himself, he can prepare himself for God's grace, or accept the offered grace, nor be capable of it for and of himself, or apply or accommodate himself thereto, or by his own powers be able of himself, as of himself, to aid, do, work, or concur in working anything towards his conversion, either wholly, or half, or in any, even the least or most inconsiderable part; but that he is the servant [and slave] of sin, John 8:34, and a captive of the devil, by whom he is moved, Eph. 2:2; 2 Tim. 2:26. Hence the natural free will according to its perverted disposition and nature is strong and active only with respect to what is displeasing and contrary to God.
Solid Declaration, FC 1580