In the natural or physical sense, a citizen is a person who legally belongs to a country either because of being born there or being declared a member by law. Consequently they have the rights and protection of that country or sovereignty but also duties or obligations required of them.
We will be either citizens of heaven or hell – the choice is ours
allegiance from being citizens of the kingdom of Satan to that of God and become (future) citizens of heaven (Eph 2:1-5; Phil 3:20; Col 1:13). By repenting of sin and receiving Christ we are born-again by faith into the Kingdom of Heaven (Mt 3:2; Jn 1:12, 3:3,36). It is our responsibility as new creatures in Christ (our new master), to live in accordance to our heavenly citizenship, not our earthly citizenship for not all who think they are saved actually are (Mt 7:21-27; Rom 6:22; 2 Cor 5:17; Gal 5:22,23; 2 Pet 1:4).
Although as Christians we are still in this world we are not to be dictated to or controlled by its ungodly values, instead focused on the heavenly realm we will be going to and so endeavour to get in some practice now living by its values (Jn 17:14-16; Rom 8:13; Col 3:2-5). The Holy Spirit who comes to live in us begins to transform our worldly desires into those that please God and bring us into conformity to Christ (Rom 8:29, 12:1,2; 1 Cor 3:16, 6:19,20).
While we are still in this world we are ambassadors of the heavenly realm and should be interacting with the lost as "salt and light" with the intention of getting them to join us there (Mt 5:14-16, 28:18-20). We should be role models of God's original design for humanity (before sin blighted us), of being blameless in His sight (Eph 1:4; Phil 2:15; 2 Pet 3:14). There should be a marked contrast between our responses and lifestyle compared to those who don't know the transforming power of the salvation available in Jesus (Mal 3:18; Eph 4:17-24). Ours should reflect that we love God with our whole being and this outworked in our whole manner of life through our actions and work ethics, our words of gratitude and encouragement, not criticism and complaining, because we focus on the next (eternal) world and not the temporary visible life here on earth and sow to please the Spirit, not the flesh (Mk 12:30; 2 Cor 4:18; Gal 6:8).
See also: born-again, difference, divine exchange, Kingdom of God, Kingdom of Satan, lifestyle, master, response.