1 Peter 1:1-2
It is the life that is obedient to God and separated from the world that provides the proof of one's conversion. If the Christian is legally cleared of guilt before God and obedient to Him, he no longer "belongs" to the world; the Bible no longer perceives such a person as being "in the flesh."
Philippians 3:20 offers understanding of another separation from the world: "For our citizenship is in heaven, from which we also eagerly wait for the Savior, the Lord Jesus Christ." His spiritual separation produces for the Christian a legal transfer of citizenship that he must recognize.
Colossians 1:12-13 confirms this: "Giving thanks to the Father who has qualified us to be partakers of the inheritance of the saints in the light. He has delivered us from the power of darkness and conveyed us into the kingdom of the Son of His love." As a result of these separations, the Christian must live his life as a stranger and pilgrim as if in a foreign land, obeying the laws of his new nation by placing higher priority in his activities as a citizen of the Kingdom of God.
This opens the door to another line of practical thought, conduct, and attitude: "Do you not know that friendship with the world is enmity with God? Whoever therefore wants to be a friend of the world makes himself the enemy of God" (James 4:4). We normally do whatever we can to avoid our enemies, even to the point of fleeing from them if necessary. This reality should help us to understand why God commands us:
Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness? . . . Therefore "Come out from among them and be separate, says the Lord. Do not touch what is unclean, and I will receive you." (II Corinthians 6:14, 17)
It is by means of conduct motivated by the Holy Spirit that we are to come out from among unbelievers and be separate. We cannot—we must not—straddle the fence; we cannot serve two masters. Once we are called, we must serve God, or we will have received God's grace in vain (II Corinthians 6:1).
John W. Ritenbaugh
The Christian Fight (Part Seven)

Writing about AD 65, Peter reminds Christians of who they are, a special people known personally by God. His major theme is strengthening the feeble knees of brethren buckling under their burdens by reminding them of their hope. This hope rests on election, meaning "those sought out," for God sought them out purposely to acquaint them with Himself. Because believers are spiritually separated from the world, their citizenship transferred into God's Kingdom, they must live as strangers and pilgrims in a foreign land, obeying their new nation's laws and coming out from among unbelievers. Friendship with the world is enmity with God. Guarded and kept by God's power, they are foreigners scattered through Pontus, Galatia, Cappadocia, Asia, and Bithynia, awaiting their incorruptible inheritance.