Peter encourages these struggling, persecuted believers with a reminder that we are to live our lives not in the flesh, but for the will of God. John Owen once said, “sin in the believer is a burden which afflicts him rather than a pleasure which delights him.” His statement recognizes that even after being born from above when we were regenerated, we will continue to struggle with sin and temptation.
Believers have been given a new nature in Christ, which desires to live in righteousness and obedience to God. But the conflict between the flesh and the spirit is real and it continues while we are in this earthly tent.
Any ability we have to be victorious over the old self is only accomplished through Christ's victory over sin. Through Christ's death and resurrection, believers are freed from the bondage and dominion of sin, although we still struggle with it. 
We are to arm ourselves as our Savior armed Himself with the same purpose.