There is nothing an individual has left to atone FOR.
After Christ's all-sufficient atonement, my own personal efforts to atone "just a little bit more" are like a few drops of water in an overful cup of water--they'll just overfill the cup. Christ filled the cup of forgiveness right up to the very brim.
Adding more to it is useless, and demeans His atonement for us, treating it as, well...insufficient.
But what did Our Lord mean when he said, "If anyone wants to be a follower of mine, let him renounce himself, take up his cross, and follow me." ? Matt 16:24
And what did St. Paul mean when he said, "Now I rejoice in what was suffered for you, and I fill up in my flesh what is still lacking in regard to Christs afflictions, for the sake of his body, which is the church." ? Col 1:24