Since it is upon God’s command that the work must be undertaken, it is God’s command that gives one the assurance that the work will accomplish its purpose. Looking at matters by themselves, it would be worse than useless to undertake reasoning with unbelievers. But it is the deep conviction of the total depravity of man that makes one throw his whole reliance upon God in all respects, and not the least in this question of reasoning with unbelievers. It is only he who deeply believes in the total depravity of man that can really preach with conviction that his work will not be in vain. Since he is convinced that the ethical alienation has been against God and against nothing else, he also knows that God is able to remove the ethical alienation. He, therefore, trusts that the Holy Spirit to whom, in the economy of redemption, the task has been assigned of convicting the world of judgment, will use the means of rational argumentation to accomplish his task. This hope is not inconsistent with the conception of the immediacy of the work of the Holy Spirit. That immediacy is complete. Our arguments taken by themselves effect nothing, while the Holy Spirit may very well convict without the use of our argument as he may convict without the use of our preaching. Yet because God is himself a completely rational God and has created us in his image, there is every reason to believe that he will make argumentation effective.
– Cornelius Van Til, A Survey of Christian Epistemology p.196-197