And straightway the father of the child cried out, and said with tears, “Lord, I believe; help Thou mine unbelief.”-Mark 9:24.

When a man first lays hold upon Jesus he is very apt to be in distress, if his joy be not always at its full height (or) he is untrained in spiritual conflict, and easily dismayed (or) the tremor of his former conviction is upon him, and he is prone to relapse into it. The light which he has received fills him with intense delight, but it is not very clear and abiding; he sees men as trees walking, and is ready to conjure up a thousand fears. The weakness of newborn faith, therefore, calls for the compassion of all who love the souls of men.

This man did well in looking for the help against his unbelief to the right quarter. He did not say, “Lord, I believe; and now I will try to overcome my unbelief.” No; but “Lord help,” as if he felt that the Lord alone could do it. No physician can cure unbelief but Christ. He is the physic for it, and He is the physician too. If thou hast any unbelief, take thou the blood of Christ to cure it with. Think of Him, God in the glory of His person, tabernacling among men, working out a perfect righteousness, dying a felon’s death upon the cross in the sinner’s stead; think of Him as rising from the dead, no more to die: think of Him as ascending into heaven amidst the shouts of angels: think of Him as standing at the right hand of God with the keys of death and hell at His girdle: think of Him as always pleading the merit of His blood before the Father’s throne; and, as thou considerest concerning Him, in the power of the Spirit, thine unbelief will die, for thou wilt say, “Lord, the thought of Thee has helped mine unbelief; while I have been studying Thee, and feeding my soul on Thee, and making Thee to be as bread and wine to my soul, my unbelief has gone. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/1033.cfm