What power does human reason hold, in regard to the kingdom of God and spiritual discernment? Calvin breaks down the kingdom of God and spiritual discernment into three things: knowledge of God, knowledge of His paternal favor toward us, and the method of regulating our conduct in accordance with the divine law, and examines human reason in light of them:
Knowledge of God & God's Paternal Favor Towards Us
The short and sweet of it is this, that human reason simply can not begin to reach the level of understanding God and His Paternal Love for us alone. John 1:4-5 says, "In Him was life; and the life was the light of men. And the light shineth in darkness; and the darkness comprehended it not." The human soul is indeed "irradiated with a beam of divine light, so that it is never left utterly devoid of some small flame, or rather spark, though not such as to enable it to comprehend God. And why so? Because its acuteness is, in reference to the knowledge of God, mere blindness." We must have help through the Holy Spirit to be able to grasp the knowledge and love of God. As we see from Moses that even the most visual proof is not enough to overcome our fallen "human reason", "while upbraiding the people for their forgetfulness, at the same time observes, that they could not become wise in the mysteries of God without his assistance. 'Ye have seen all that the Lord did before your eyes in the land of Egypt, unto Pharaoh, and unto all his servants, and unto all his land; the great temptations which thine eyes have seen, the signs, and these great miracles: yet the Lord has not given you a heart to perceive, and eyes to see, and ears to hear, unto this day'."
"No, is not He Himself the living image of His Father, in which the full brightness of His glory is manifested to us? Therefore, how far our faculty of knowing God extends could not be better shown than when it is declared, that though his image is so plainly exhibited, we have not eyes to perceive it. What? Did not Christ descend into the world that he might make the will of His Father manifest to men, and did he not faithfully perform the office? True! He did; but nothing is accomplished by his preaching unless the inner teacher, the Spirit, open the way into our minds. Only those, therefore, come to him who have heard and learned of the Father. And in what is the method of this hearing and learning? It is when the Spirit, with a wondrous and special energy, forms the ear to hear and the mind to understand."
Method of Regulating Our Conduct
Man has to some extent the ability to regulate our conduct based on the engraving of divine law on the heart. This is ability can be seen when looking at the abstract level, homicide is an evil, "no man will deny," yet it has its flaws in that at the say time "one who is conspiring the death of his enemy deliberates on it as if the thing was good."
"This is correctly termed the knowledge of the works of righteousness, a branch in which the human mind seems to have somewhat more discernment than in the former two, since an apostle declares, 'When the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: which show the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the meantime accusing or else excusing one another.' If the gentiles have the righteousness of the law naturally engraved on their minds, we certainly cannot say that they are altogether blind as to the rule of life. Nothing, indeed, is more common, than for man to be sufficiently instructed in a right course of conduct by natural law, of which the apostle here speaks."
It comes down to us ultimately needing God for everything.