Upon believing in Christ they became God's new people and their citizenship was transferred from this world and its systems and beliefs to God's kingdom. As a result they live as aliens or foreigners not as citizens, they live differently and that living provokes reactions. It is against that background that Peter writes of the need to "abstain from sinful desires, which war against your soul"(2:11), and in 4:1 of the need to "arm yourselves also with the same attitude [as Christ]". The warfare imagery runs throughout Peter's letter because that is reality as it really is.
In battle we expect hardships and sufferings, in battle we are constantly on our guard or alert (1:13, 4:7h, 5:8). In a way in which we are not in peacetime. It is a call we need to hear and heed - we are in a battle, the world is not our friend it is our enemy, the supposed tolerance of society isn't tolerance of truth, our friends lack of hostility to the gospel is not a good thing it is a sign of the sheer scope of the for souls we face!
Stand fast, stand ready for battle, feet planted on the true grace of God.