What happens to the body at the resurrection if it is still corrupt and prone to sin now?
The present body remains in the flesh, corrupt, prone to sin and death, but the joy of resurrection is that we will have a new body. This same concept Paul adheres to, as taught in 1 Corinthians. It will not remain in its current corrupt state.
Les teaches that the body is still in the flesh, still corrupt, still prone to sin and death. But the beauty and joy of resurrection is that we are going to have a new body, and he notes this was covered in detail back in 1 Corinthians. Paul adheres to this same concept in verse 22, where he speaks of God sealing us and giving the earnest, the down payment, of the Spirit in our hearts. The surrounding context also shows Paul's pastoral concern for the Corinthians, not lording over their faith but being helpers of their joy, for by faith they stand.