Mark 9:23

If the man believed, as he said, why does he add, help my unbelief? It may be answered: because faith is manifold; their is a faith of beginners, and a faith of the perfect. The incipient faith this man already possessed, and he besought our Saviour to help him to the higher degrees of this virtue. No one becomes great and perfect all at once, but must first set off with small beginnings, and thus gradually ascend to the height of perfection. Thus the man, who, by the inspiration of grace has received imperfect faith, may be said at the same time to believe, and still to be incredulous. (Ven. Bede) — Here we are taught that our faith is weak, and has need of support and increase from God’s assistance. When tears accompany our faith, they obtain for us the grant of our petitions. (St. Jerome)

VI. The answer Christ gave to his address (v. 23); If thou canst believe, all things are possible to him that believeth. Here, 1. He tacitly checks the weakness of his faith. The sufferer put it upon Christ's power, If thou canst do any thing, and reflected on the want of power in the disciples; but Christ turns it upon him, and puts him upon questioning his own faith, and will have him impute the disappointment to the want of that; If thou canst believe. 2. He graciously encourages the strength of his desire; "All things are possible, will appear possible, to him that believes the almighty power of God, to which all things are possible;" or "That shall be done by the grace of God, for them that believe in the promise of God, which seemed utterly impossible." Note, In dealing with Christ, very much is put upon our believing, and very much promised it. Canst thou believe? Darest thou believe? Art thou willing to venture thy all in the hands of Christ? To venture all thy spiritual concerns with him, and all thy temporal concerns for him? Canst thou find in thy heart to do this? If so, it is not impossible but that, though thou has been a great sinner, thou mayest be reconciled; though thou art very mean and unworthy, thou mayest get to heaven. If thou canst believe, it is possible that thy hard heart may be softened, thy spiritual diseases may be cured; and that, weak as thou art, thou mayest be able to hold out to the end.

Verses 14-29 The father of the suffering youth reflected on the want of power in the disciples; but Christ will have him reckon the disappointment to the want of faith. Very much is promised to our believing. If thou canst believe, it is possible that thy hard heart may be softened, thy spiritual diseases may be cured; and, weak as thou art, thou mayest be able to hold out to the end. Those that complain of unbelief, must look up to Christ for grace to help them against it, and his grace will be sufficient for them. Whom Christ cures, he cures effectually. But Satan is unwilling to be driven from those that have been long his slaves, and, when he cannot deceive or destroy the sinner, he will cause him all the terror that he can. The disciples must not think to do their work always with the same ease; some services call for more than ordinary pains.