"A man that is fallen into a pit cannot be supposed to help himself out of it, but by one of two ways; either by doing all himself alone, or taking hold of, and improving, the help offered him by others. Likewise an unconverted man cannot be supposed to help himself out of his natural state, but either in the way of the law, or covenant of works, by doing all himself without Christ; or else in the way of the Gospel, or covenant of grace, by exerting his own strength to lay hold upon, and to make use of the help offered him by a Saviour.
But, alas! the unconverted man is dead in the pit, and cannot help himself either of these ways; not the first way, for the first text (Rom 5:6) tells us, that when our Lord came to help us, 'we were without strength,' unable to recover ourselves. We were ungodly, therefore under a burden of guilt and wrath, yet 'without strength,' unable to stand under it; and unable to throw it off, or get from under it: so that all mankind would have undoubtedly perished, had not 'Christ died for the ungodly,' and brought help to those who could never have recovered themselves. But when Christ comes and offers help to sinners, cannot they take it? Cannot they improve help when it comes to their hands?
No, the second text (John 6:44) tells us, they cannot; 'No man can come unto me,' that is, believe in me, 'except the Father draw him.' This is a drawing which enables them to come, who till then could not come; and therefore could not help themselves by improving the help offered.
It is a drawing which is always effectual; for it can be no less than 'hearing and learning of the Father,' which, whoever partakes of, come to Christ (verse 45). Therefore it is not drawing in the way of mere moral suasion, which may be, yea, and always is ineffectual. But it is drawing by mighty power (Eph 1:9), absolutely necessary for those who have no power in themselves to come and take hold of the offered help." --Thomas Boston, from Human Nature in its Fourfold State (Banner of Truth, 1964, pp. 183-184)