In the book The Heart of the Cross, Philip Graham Ryken writes this about the offensive truth of the cross:
What is offensive about the cross is that it utterly contradicts this line of reasoning [that people are good enough to get into heaven]. Consider what the cross says. It says you are not righteous. It says you are a sinner. It says you do not measure up to God's perfect standard. The reason it was necessary for Jesus to be crucified was because of the sins of humanity, your own sins included.
The cross also says you are helpless. It says you cannot get into heaven on the strength of your own record. You need someone else to offer his perfect life on your behalf. This is one difference between Christianity and every other religion. Every other religion tells you to present to God the best record you can. Christianity teaches that God offers his perfect record to you. But that means you cannot gain eternal life without his total help. The death of Christ on the cross proves the helplessness of humanity.
Then the cross says you are hopeless. The cross of Christ shows that sin deserves the wrath and curse of God. It proves that without Christ, sinners will perish for their sins. Don't you think that if God could have saved you any other way he would have done it? But there was no other way, because every sin deserves a hell of suffering.
Who wants to be told that he or she is helpless, hopeless, and unrighteous? No one does. The very idea is offensive. Most people are convinced of their own basic goodness. They go through life helping themselves and hoping for the best. These attitudes make the cross offensive to the moral individual. The cross is a warning that you are dead in your sins. It announces that you cannot help yourself, indeed that you are without hope.
Grace upon grace,