Peter lovingly addresses his readers, earnestly urging rather than forcefully demanding. His entreaty is given because he wants what is best for these beloved Christians. He wants them to do what is best so that their lives can be the best as they ought to be.
Peter addresses his readers as aliens and strangers, which is the basis for his urging. The action he is encouraging is reasonable based on the realization of who they really are. The two terms share one Greek root, while each includes a second root that creates the distinction between the two words. Alien deals with the dwelling itself, or by implication the family. In other words, the family unit or home is now located in a place that is not its origin. Stranger refers to making one's home or residing. Beyond the fact of having one's house in a foreign land, it is the idea of settling in there and realizing that one is now living in a foreign land, probably never to return to his original home. The reality that the believers are aliens and strangers, not really belonging where they are, is the reason they are to act and live as Peter is about to encourage.
The concept is quite familiar to his readers. Peter had already referred to them as strangers in the first verse of his epistle. In 1:1, however, he was referring to earthly geography. These believers had been forced from their homes in the Diaspora. They were now living in various regions which are listed in the epistle's opening. They know exactly what it is like to be displaced and to live in a strange place. In 2:11, Peter is applying the same concept in the spiritual realm.
Peter is stating that this world is not the true home of these (or any) believers. These readers, who understood the concept quite well due to their geographic displacement, are to apply that understanding to their spiritual lives. They are now residents of heaven, holding heaven's culture and values. This world's cultures and values are foreign. A Latin would feel out of place in an African culture and would not participate nor be interested in certain practices. A diplomat in a foreign country might explore various cultural practices, but he might never understand them or embrace them for himself, even though he lives in the country where they are practiced. This is exactly how a Christian should be toward this world.
Because of their foreign status in this world, Peter urges the believers to abstain from the fleshly lusts associated with it. They are to hold themselves off from such things and not let themselves go toward them. Specifically, the fleshly lusts are the longings and desires associated with this world. They are urges that are bodily, temporal, and unregenerate. "For all that is in the world, the lust of the flesh and the lust of the eyes and the boastful pride of life, is not from the Father but is from the world" (I John 2:16).
Peter's basis for urging this response is completely logical. Because these believers are aliens, it is reasonable and expected that they would not embrace the practices of the corrupt world in which they are living as non-citizens. Various religious groups have recognized this danger in the past. The Pilgrims, for example, had left England and settled for a time in Holland. In Holland they feared the world's influence as they saw the culture damaging their children in terms of worldliness and corruption. The Pilgrims ended up leaving Holland as a result. While a geographic move may not be necessary, or even effective, the underlying concern is correct. Earthly, sinful, basely passionate, self-centered, and proud desires have nothing to do with Christianity. Instead they are the true manifestation of a world without God. No Christian should embrace those things.
Peter goes on to tell why his instruction is so important. The fleshly lusts are dangerous and should be avoided, never embraced, because they wage war against the soul. Experimenting with or involving oneself in those lusts is asking for trouble and inviting conflict. Fleshly lusts, once embraced or experimented with, create a raging conflict. The war already exists, but it is folly to make the war harder by inviting the enemy into one's own camp. The wisest action is to avoid the battle as much as possible and to have good defenses so that the enemy doesn't have a good opportunity. Embracing (failure to abstain from) fleshly lusts is deliberately causing the battle to rage.
This battle is quite serious. Although a battle dealing with fleshly lusts would seem to be a physical battle, it is actually a spiritual battle. Satisfying the flesh carries the battle into the arena of the soul. "For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world forces of this darkness, against the spiritual forces of wickedness in the heavenly places" (Ephesians 6:12).
Satan is at the root of the world's system and its fleshly lusts, and he wants to destroy Christians. Christians must respond soberly, seeking to limit the temptation and pull of the world. They should never (intentionally or unintentionally) make the battle harder by deliberately embracing something that earnestly seeks to destroy his soul. Many things about the world hold some appeal, and they are readily embraced by the citizens of this world, but they should not be part of the lives of believers, whose residence here is that of strangers.