Why was grace the only thing that could save the Corinthians?
Grace alone could save the Corinthians because their city was wracked with immorality and they accepted it as part of society. Anything other than grace would never have succeeded; only by grace did God begin to save those wicked Corinthians, bringing them into salvation as babes in Christ.
Les explains that Paul came into Corinth with the gospel of the grace of God. Corinth was a city wracked with immorality, and its people thought nothing different of it because it was part of their whole society. In that setting, anything but grace would have never succeeded; it was only by grace that God began to save these wicked Corinthians. Les also notes that the spiritual life parallels 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 a babe in Christ, so there will be failures, just as an infant is not born trained.