There is nothing to prevent men from obeying the will of God but their own depraved dispositions, and aversion to the things of God. The natural faculties of men would be sufficient to enable them to do what he commands, if they employed them properly. If they employ them otherwise, the fault rests exclusively with themselves. And as the corruption of our nature does not deprive a man of his natural faculties, or of perfect liberty to act conformably to the decision of his own mind, the obligation under which he lies to do right continues in full force. From this we see, first, how justly God punishes men for their crimes, who, unless inclined and enabled by his grace, cannot liberate themselves from the slavery of sin; and, further, that the inability of men to obey God, not being natural but moral inability, cannot deprive God of the right to command obedience, under the pain of his most awful displeasure.
On this subject the distinction between natural and moral should always be kept in view. Natural inability consists in a defect in the mind or body, which deprives a man of the power of knowing or doing anything, however desirous he may be of knowing or doing it. Natural inability, then, can never render a man criminal. Moral inability consists in an aversion to anything, so great that the mind, even when acting freely, that is, without any external impulse or constraint, cannot overcome it. When this aversion exists as to what is good, it is inseparable from blame, and the greater this aversion is, the greater is the criminality. All men are daily accustomed to make these distinctions, and according to this rule they constantly form their opinion of the conduct of others.
In the nature of things it is impossible that the justice of God can ever demand of reasonable creatures less than perfect obedience. To say that the moral inability of man to obey the law of God destroys or weakens in the smallest degree his obligation to obey that law, is to add insult to rebellion. For what is that moral inability? It is, as has been observed, no other than aversion to God, the depraved inclination of the carnal mind, which not only entertains and cherishes enmity against God, but is itself that enmity. And let it not be said that the view the Scriptures give of the natural depravity of men, and of the sovereign and efficacious grace of God, reduces them to the condition of machines. Between men and machines there is this essential difference, and it is enough for us to know, that man is a voluntary agent both in the state of nature and of grace. He wills and acts according to his own dispositions, while machines have neither thought nor will. As long, then, as a man’s will is depraved and opposed to God, his conduct will be bad; he will fulfill the desires of the flesh and of the mind; and on the other hand, when God gives the sinner a new disposition, and a new spirit, his conduct will undergo a corresponding change. “The liberty of a moral agent consists in the power of acting conformably to his choice. Every action performed without external constraint, and in pursuance of the determination of the soul itself, is a free action. The soul is determined by motives; but we constantly see the same motives acting diversely on different minds. Many do not act conformably to the motives of which they yet acknowledge all the force. This failure of the motive proceeds from obstacles opposed by the corruption of the heart and understanding. But God, in giving a new heart and a new spirit, takes away these obstacles; and in removing them, far from depriving a man of liberty, he removes that which hindered him from acting freely, and from following the light of his conscience; and thus, as the Scriptures express it, makes him free. The will of man, without divine grace, is not free but enslaved, and willing to be so.”
Is it objected, that if a man be so entirely corrupt that he cannot do what is right, he should not be blamed for doing evil? To this it is sufficient to reply, that if there be any force in the objection, the more a voluntary agent is diabolically wicked, the more innocent he should be considered. A creature is not subject to blame if he is not a voluntary agent; but if he be so, and if his dispositions and his will were absolutely wicked, he would certainly be incapable of doing good, and according to the above argument he could not be blamed for doing evil. On this ground the Devil must be excused, nay, held perfectly innocent in his desperate and irreconcilable enmity against God. A consequence so monstrous totally destroys the force of the objection whence it is deduced. But if the objection be still pressed, if any one shall proudly demand who hath resisted his will, why hath he made me thus? the only proper answer is that of the Apostle, ” Nay but, man, who art thou that replies against God?”
~Robert Haldane, Exposition of the Epistle to the Romans (New York: Robert Cater, 1847), 339-343; Romans 8:7. [Some spelling modernized; italics original; and underlining by post’s author as cited below.]
Source: http://www.puritanboard.com/showthread.php/89079-illustrations-of-moral-inability-%28Man-WILL-not-come-rather-than-merely-CANNOT-come%29, Comment #1