Those who think this verse, 1 Corinthians 1:26, means Christians are not wise are skipping over the words “after the flesh.” There are different types of wisdom. There is the wisdom of this world (“after the flesh”), and there is the wisdom of God. The message of this passage is that God has chosen a seemingly foolish message, requiring humility and childlike faith, to confound those proud folks who think they are wise.
If you look around the Christian faith, you won’t find many of those proud people who are puffed up in their own “fleshly” wisdom. The same thing is true with 1 Cor. 3:18: it speaks of anyone who “seems to be wise in this age,” and goes on to explain that “the wisdom of this world is foolishness with God.” Again, this is referring to those who are wise in their own eyes—they only think they are wise. So the atheist isn’t wiser than the dumbest of Christians. Rather, according to the Bible, the atheist is a fool (see Psa.14:1).
1 Corinthians 1:26 For you see your calling, brethren, that not many wise according to the flesh, not many mighty, not many noble, are called.