‘Thou fool!’ We don't use such terms with people, but what he means is, ‘You who are a minority at Corinth there, you count yourself as members of the Christian church, and yet you don't believe in the resurrection? You fool: to prefer your pagan philosophy above the teaching of God and of Christ.’ It was indeed foolish.
A Christian is someone who has been converted. A Christian has had his eyes opened by God; he has been illuminated by the Holy Spirit. He was not a believer, but because the Spirit has worked in a regenerating way in his heart, and he has come to realise and know instinctively that the Christian gospel is true. The truth of the gospel has convicted him, and he has yielded to Christ. Then he has found proof after proof because his life has been changed and he is filled with love for God and gratitude to Christ for dying on Calvary. A miracle has taken place within him. The sins he loved, he is now very uncomfortable with, and he can't easily practice them anymore. If he sins he is in pain, and his conscience is troubled and he has to repent, and he longs to be more righteous. What has happened to him? A miracle has taken place within him. He has been changed by the power of God. He has tasted the power of God. And he has gained great understanding. He has got a spiritual family; he relates to Christian people, and he loves fellow believers and talks with them more than anyone else. The great miracle of conversion! All these things make it so much easier for him to believe what God has already shown him is true: that Christ is the incarnate God, and that he suffered and died for sinners and has risen from the dead, and there is a resurrection ahead for all who trust in him.