If I were to narrow it down to one indicator that someone is an unbeliever, I would pick foolishness. Foolishness in the Bible is contrasted with wisdom. Wisdom orients itself around the fear of the Lord, seeks to interpret life from the perspective of God, and looks to live life in reference to God.  Foolishness, in contrast, makes self the point of reference. It does what is right in its own eyes.

If you were to pick one telltale sign that someone is not of Christ, what would it be?

How about love? John in his first epistle points to love as a mark of the believer. Would an unbeliever exhibit its opposite? That makes sense but what is the opposite of love? Besides, many unbelievers far outshine Christians in giving and sacrifice.

Maybe faith? By definition, a believer is someone who has faith, someone who believes. An unbeliever does not have faith.

True, but it’s not that simple. Churches are populated by plenty of people who have faith to align themselves with Christianity. But that does not mean they have saving faith the rests in Christ alone for their salvation, or a faith that fuels the way they conduct their lives with Christ as the point of reference and dependence.

Plus, unbelievers do have faith, just not faith in Christ as He is revealed in Scripture and found in the gospel. Many “spiritual” people profess some sort of faith – in something.  So, as with love, faith is a little tricky to discern as the mark of one without Christ.

If I were to narrow it down to one indicator that someone is an unbeliever, I would pick foolishness. Foolishness in the Bible is contrasted with wisdom. Wisdom orients itself around the fear of the Lord, seeks to interpret life from the perspective of God, and looks to live life in reference to God.  Foolishness, in contrast, makes self the point of reference. It does what is right in its own eyes.

Foolishness leads people to deny God and suppress the truth in unrighteousness. Though they claim to be wise, they are fools (Rom. 1:22).  By divine assessment, their “wisdom” is foolishness. First Corinthians, chapters one and two, provide a running start to this analysis: “But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned” (1 Cor. 2:14).

For those marked by foolishness, the cross of Christ is nonsensical and unnecessary, particularly for those who have fashioned a “god” in their own image, to suit their own preferences for a deity palatable to them.

Unregenerate people, even religious ones, even spiritual ones, are marked by foolishness that rejects God as He reveals Himself and rejects the revelation of His truth given in His Word, the Bible.  Even if they agree with something, it is because they have decided to embrace it from their position as arbiters of right and wrong, truth and error, good and bad, rather than think God’s thoughts after Him.

But foolishness is not simply the mark of the unbeliever. Foolishness is also the mark of unbelief.

If unbelievers are marked by foolishness and believers are marked by wisdom, why do we see so many Christians looking and acting like unbelievers? Why are the gospels and the epistles of the New Testament so full of instruction to believers not to act in foolishness, not to act like unbelievers?

We are constantly told to walk in the way of wisdom, rather than conduct ourselves in foolishness.  At the end of the Sermon on the Mount, with all its explanations and ethical admonitions, Jesus says that the wise man is he who build his house on the rock, while the foolish man builds his house on the sand. What distinguishes the two foundations?—hearing Christ’s word and putting it into practice.

James says that we are foolish when we believe something contrary to what the Bible teaches (Jas. 2:20). Paul echoes the sentiment when we hold beliefs that lead us astray from the truth God has given (Gal. 3:1-3).

So when believers hold doctrine deviant from Scripture or practice behavior deviant from Scripture, they are acting the fool. They are displaying the mark of unbelief.  That’s why we find the constant tug of discipleship back to wisdom.  For example, “See then that you walk circumspectly, not as fools but as wise, redeeming the time, because the days are evil. Therefore do not be unwise, but understand what the will of the Lord is” (Eph. 5:15–17).

Foolishness is not always malicious. Sometimes it is an expression of weakness. Hudson Taylor, the great missionary to China, diagnosed his depression as primarily consisting in unbelief.  He saw himself not taking hold of the gospel, but allowing doubts and fears to rule the day. The foolishness that is bound up in the heart of the unregenerate from their mother’s womb is not eradicated by the new birth.  It continues to exert its rebellious influence as long as we are in this mortal body.

Unbelievers have not cornered the market on foolishness. Though foolishness is the mark of the unregenerate, it also characterizes unbelief, something Christians wrestle with daily.  Hence the prayer: “I do believe; help me in my unbelief.”

Stan Gale is a minister in the Presbyterian Church in America, and is the author of the newly released book, A Vine-Ripened Life: Spiritual Fruitfulness through Abiding in Christ. This article appeared on his blog and is used with permission.