Forasmuch then as Christ hath suffered for us in the flesh - Since he as a man has died for us. See the notes at 1 Peter 3:18. The design was to set the suffering Redeemer before them as an example in their trials.
Arm yourselves likewise with the same mind - That is, evidently, the same mind that he evinced - a readiness to suffer in the cause of religion, a readiness to die as he had done. This readiness to suffer and die, the apostle speaks of as armour, and having this is represented as being armed. Armour is put on for offensive or defensive purposes in war; and the idea of the apostle here is, that that state of mind when we are ready to meet with persecution and trial, and when we are ready to die, will answer the purpose of armour in engaging in the conflicts and strifes which pertain to us as Christians, and especially in meeting with persecutions and trials. We are to put on the same fortitude which the Lord Jesus had, and this will be the best defense against our foes, and the best security of victory.
For he that hath suffered in the flesh hath ceased from sin - Compare the notes at Romans 6:7. To “suffer in the flesh” is to die. The expression here has a proverbial aspect, and seems to have meant something like this: “when a man is dead, he will sin no more;” referring of course to the present life. So if a Christian becomes dead in a moral sense - dead to this world, dead by being crucified with Christ (see the notes at Galatians 2:20) - he may be expected to cease from sin. The reasoning is based on the idea that there is such a union between Christ and the believer that his death on the cross secured the death of the believer to the world. Compare 2 Timothy 2:11; Colossians 2:20; Colossians 3:3.