“Now if we have died with Christ, we believe that we will also live with him.” -Romans 6:8
Paul repeats what he said earlier (vs.4) that he might set the stage for what follows, namely that Christian must pursue the newness of life while they remain in their earthly tabernacle the same way they mortify sin in the body as long as they live in this body. As Christ died and rose again once and for all—never to die again—so will we and so do we. We too will be resurrected but it is also true that our pursuit of mortifying sin and living out the newness of our lives must never die.
One modern metaphor in which the proverbial rubber meets the road concerning Paul’s point is to think about a car without brakes traveling up a hill. There is no place on the hill of our sanctification where we can let off the accelerator without it resulting in a tragic reversal and crash. No to say, one could lose his salvation. That is not the case. It is to say that if one returns to his old life, he was none of Christs and Christ was none of his. To live according the the old nature is to deny Christ.