Do I think that God, in the ordinary course of his providence, grants this assistance in an irresistible manner, or effects faith and conversion without the sinner’s own hearty consent and concurrence? I rather chose to term grace invincible than irresistible. For it is too often resisted even by those who believe; but, because it is invincible, it triumphs over all resistance when He is pleased to bestow it. For the rest, I believe no sinner is converted without his own hearty will and concurrence. But he is not willing till he is made so. Why does he at all refuse? Because he is insensible of his state; because he knows not the evil of sin, the strictness of the law, the majesty of God whom he has offended, nor the total apostasy of his heart; because he is blind to eternity, and ignorant of the excellency of Christ; because he is comparatively whole, and sees not his need of this great Physician; because he relies upon his own wisdom, power, and supposed righteousness. Now in this state of things, when God comes with a purpose of mercy, he begins by convincing the person of sin, judgment, and righteousness, causes him to feel and know that he is a lost, condemned, helpless creature, and then discovers to him the necessity, sufficiency, and willingness of Christ to save them that are ready to perish, without money or price, without doings or deservings.
John Newton, The Letters of John Newton – To the Rev. Thomas Scott p. 264.