Peter would have his readers think of a soldier dressed for battle. He is aware that the prospect of suffering can be a powerful temptation against obeying God and doing good to another (recall the contrast in 1 Peter 3:11). To protect oneself against that temptation, the Christian needs to dress himself in fitting armour. That armour is “the same way of thinking.” Peter has arm
as an imperative, to express that his readers must put on this armour.
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,