When you finish your course of studies at a medical school, you are awarded a degree that allows you to practice medicine. The degree is a testimony to your accomplishment. What you do with that accomplishment is thereafter up to you. You may proceed to practice medicine, or you may move on to a career in laying bricks. What you do from thence forward has no impact on your accomplishment. You will always have your degree. That part is done. It is finished.
As Christ himself testified, the cross accomplished his work. What was his work? The forgiveness of our sins and our reconciliation with God. That part is done. It is finished.
Nothing we do or fail to do thereafter has any effect on that accomplishment, any more than the certified doctor’s choice of profession has any effect on the certification itself.
The only difference is that the doctor personally earned the degree, where Christ earned our spiritual accreditation on our behalf. The medical degree is a qualification; the spiritual degree is a gift.
It is a grace to be aware of that gift, to believe in it. But the belief doesn’t affect our possession of it.
Most of Christ’s instruction has to do with how best to make use of our spiritual certification, how to practice medicine rather than lay bricks.