Why was grace the only way the wicked Corinthians could be saved?
Grace was the only way because anything but grace would never have succeeded. The Corinthians were wracked with immorality and thought nothing different of it, so only by grace did God begin to save these wicked Corinthians. Paul came to them with the gospel of the grace of God.
Les explains that Corinth was a city wracked with immorality, and its people were part and parcel of that whole society, thinking nothing different of it. Into that setting Paul came with the gospel of the grace of God. Les says anything but grace would never have succeeded, but it was only by grace that God began to save these wicked Corinthians. He then draws a parallel between the spiritual life and the physical life: no one is born into the family of God as a full, mature spiritual person. Everyone comes into a salvation experience as babes in Christ, so there will be failures. Just as an infant is not trained immediately, new believers must grow, and we cannot expect them to be fully mature right away. The old evangelist illustration about diapers hanging on clotheslines shows how you could tell a home had a baby—spiritual infants likewise need growth.