One of the obvious demonstrations of God’s common grace is human reason, “the power of the mind to think, understand, and form judgments by a process of logic” (OED). But for the impaired, it is a gift we all employ on a daily basis, and much of what we believe is based on this God-given reason.
There are, however, truths that may not be discerned by reason alone. For example, Paul writes, “The natural person does not accept the things of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned” (1 Cor. 2:14). The word translated “natural” may also be translated “unspiritual,” describing one who does not have the Holy Spirit, or as the NET translates it, “the unbeliever.” Just as reason is a defining characteristic of what it means to be human, the indwelling of the Holy Spirit is a defining characteristic of what it means to be a believer (Rom. 8:9).
The most intelligent yet unbelieving person may not discern “the things of the Spirit of God” apart from the Spirit of God. Things such as God’s special revelation of Scripture and being born again through faith in the Son of God are foolishness to those who have not the Holy Spirit. They don’t understand because they can’t. Of course, the Holy Spirit may work through human reason, but human reason alone cannot pierce the veil separating the spiritual and the natural. Only the Holy Spirit can and does for those whom he indwells.
To describe this distinction, Paul uses the expression “the mind of Christ.” We might say that the Christian is enabled by the Spirit to see spiritual things from the viewpoint of Christ. In other words, we are spiritually empowered to think, understand, and form judgments as the Holy Spirit guides, on matters of first importance, the things of God. This does not render our judgment infallible, but it does encourage all of us who trust in Christ to walk by his Spirit (Gal. 5:16-18).