A checkpoint? To my knowledge, Jesus didn't tell his followers "hey, I'm sacrificing myself for the sins of the world, but conditions apply".
If this sin nature, which to me sounds like another name for human nature, was so important, why didn't Jesus talk about it? The question of sin itself after the crucifixion is one that's open to interpretation.
We have Christians here that believe sin was conquered through Christ. Others believe that one can still sin, but the saved are still saved and THAT is the significance of the crucifixion. Others believe in a sin nature, or that the crucifixion was merely one step in the direction of salvation.