Incertitude is a paralysis of the soul's highest faculties. Doubt weakens by distraction. Etymologically it signifies moving in two opposite directions. It produces fluctuation, hesitation, and suspense. "A double-minded man is unstable in all his ways." He has a divine premonition that he needs "not expect to receive anything from the Lord." If he is a preacher, his announcement of the Gospel will be weak and ineffectual. He cannot speak with the assurance of a personal experience which is requisite to produce conviction.
How many preachers would multiply their efficiency and usefulness if they would kneel down by the side of Paul and repeat in faith his petition that they might be strengthened with might by his Spirit in the inner man! (Eph. iii. 14-21). It was his experience of the revelation of the Son of God in him by the Holy Ghost that made him successful as an evangelist, mighty in labors, courageous in danger, patient in sufferings, and triumphant in martyrdom. No heroism was ever born of doubt. It is only when the soul is set on fire by some great moral truth, clearly seen and firmly grasped, yea, ingrained into its very texture, that moral sublimity in effort, in sacrifice, and in speech emerges. Doubt heads no glorious company of martyrs marching to the stake.
It is customary to advise the doubter to a study of the Christian evidences, to count the towers of Christianity, and mark well her bulwarks. Such a survey has done me much good, and I commend it to all who have leisure. But there is for the mass of busy people a shorter way. Everybody cannot thoroughly master the treatises of Bishop Butler, Archdeacon Paley, and President Hopkins, and, if they could, they might die before they had got so far along as to be convinced of the truth and receive Christ as both Savior and Lord. What is the shorter way? With the New Testament open before him, even if he doubts the ability of Christ to save, let him act on the truth he does believe, however small, and kneel down with a sense of dependence on some higher power or person, and utter an honest prayer for help. Let him, if he doubts even the existence of God, begin as one bewildered skeptic did, by saying, "O God, if there be a God, save my soul, if I have a soul." Our merciful God did not disdain to answer this prayer. The Holy Spirit showed in quick succession the greatness of his sins and the surpassing greatness of his Redeemer, whom he was enabled by the same spirit to apprehend by faith as his personal Savior. Now this shorter way, which we recommend to the slave ignorant of the alphabet, we commend to skeptical sage sitting in his ample library. On your knees pray for light, and as fast as it comes follow it. "If any man willeth to do his will" — God's will — "he shall know of the teaching, whether it be of God, or whether I speak from myself," as a mere man without divine authority. Heaven and earth shall pass away before one jot or tittle, one crossing of a t or dot of an i, shall fail in this promise in the case of one who seeks with a spirit of obedience to know Jesus Christ's character and mission. No man can be an honest skeptic till he has faithfully tried this shorter way and found that it leads nowhere. This way honestly trodden brings the doubter to certainty. Says Joseph Cook, "I assert that it is a fixed natural law that when you yield yourself utterly to God, his light will stream through and through your soul."
God honors obedience because it implies faith as its root. This truth we commend to those who regard it a special excellence to be in uncertainty respecting their relation to Christ. To be void of assurance they regard as an evidence of humility; whereas it is an evidence of a very defective obedience and of an absence of total self-surrender to God. As well might the guest without the wedding garment plead that this destitution evinces superior humility. Assurance is always accompanied by humility. Thomas never felt smaller than in the moment when his risen Lord stood before him challenging him to test the reality of his body. When he was constrained to cry, " My Lord and my God," "how cheap and mean his previous doubts seemed, and how deep his self-abasement.
— edited from Jesus Exultant Chapter 11.