Peter calls on his readers to show spiritual courage. They are to resist the devil’s attacks by being firm in the faith. In order to understand what Peter means by firm in the faith you can read Paul’s illustration in Ephesians 6 about spiritual warfare (see here for some thoughts on spiritual armour).
In addition to the items in Paul’s spiritual armour, Peter reminds his readers that their experience of Satanic attack is not unique – it was ‘being experienced by your brotherhood throughout the world’ (v. 9). This is a call for sympathy because the experiences of conflict should lead us to empathise with others enduring the same.
Yet it is also a call to learn from and be challenged by the faithfulness of others. This is what the writer of Hebrews does in his eleventh chapter. It is good for us to think about those who have endured trouble for the faith, whether we have read about them or even knew them. And regarding them we know that they were taken safely through a hostile world by their faithful Master, provided they remained loyal to him.
So there are valid reasons for being confident when under threat from the devil. His roaring cannot harm us and eventually Jesus will reveal that he has already won the victory over all his enemies. If we keep our heads with regard to the roars and watch carefully for signs of the devil’s attack, we will discover that Jesus has provided what we need to resist the attacks of such a powerful spiritual opponent.