As indicated in the use of the term “since,” Peter wants his readers to recall what he had earlier written about the notion of Christ suffering in the flesh (1 Peter 3:18). At that point Peter had drawn out what Christ had achieved through his suffering (1 Peter 3:19–20). He now recollects Christ’s suffering in order to work out what consequence follows for his readers.
1 Since therefore Christ suffered in the flesh, arm yourselves with the same way of thinking, for whoever has suffered in the flesh has ceased from sin,