“Master, I have brought unto Thee my son, which hath a dumb spirit.”…And He asked his father, “How long is it ago since this came unto him? And he said, “Of a child.” – Mark 9:17,21

This man’s faith was real, because it was faith which triumphed over difficulties, difficulties which typify our own, and hence it was clearly the work of the Spirit of God, for no other will endure the trial. I shall ask thee, dear hearer, whether faith has triumphed over difficulties in thy case. For observe, his child was grievously tormented, and the malady was of long standing. When the Savior said to him, “How long hath this happened unto him?” he said, “Of a child.” Must it not have seemed, now that his son had grown older, a very unlikely thing that he should be recovered…Yet in the teeth of that the man believed that Christ could cast that long-established demon out of his son. Dear friend, thy case of sin is similar. The sins of thy youth rise up before thee now: are they not in thy bones? The sins of thine early manhood, and the sins of thy riper years, and, mayhap, the sins of thy decaying years; all these come up before thee…Crimes so long continued, evil habits so deeply rooted, can all these be overcome? O soul, if thou hast true faith, thou wilt say, “Yes, I believe that since Christ is God He can deliver me from all evil, and forgive me all sin. Even if I had lived as long as Methuselah, and had continued all that while in the vilest of transgression, yet Jesus is so mighty to save that He could deliver me in a moment.” His word is, “All manner of sin and of blasphemy shall be forgiven unto men.” Looking to those dear wounds, those founts of love and blood, I do believe, and will believe, that all my years of sin are gone as in a moment, and like thick clouds before a mighty wind are blown away never to return. Oh, this is faith, poor soul. I pray God enable thee to exercise it.~ C.H. Spurgeon

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