Summary: How we live reveals our understanding of who we are. As citizens of Heaven, Christians exhibit defined characteristics.
“Brothers, join in imitating me, and keep your eyes on those who walk according to the example you have in us. For many, of whom I have often told you and now tell you even with tears, walk as enemies of the cross of Christ. Their end is destruction, their god is their belly, and they glory in their shame, with minds set on earthly things. But our citizenship is in heaven, and from it we await a Savior, the Lord Jesus Christ, who will transform our lowly body to be like his glorious body, by the power that enables him even to subject all things to himself.” 
I am a citizen of the United States of America by birth. I am a citizen of Canada by choice. I am a citizen of Heaven by the grace of God through the New Birth. Citizenship identifies each of us in numerous ways. Citizenship reveals something of our view of life, of our commitment to ideals, of our investment in others. As twice-born Christians, we can lay claim to an eternal citizenship. The Christian is a citizen of an unseen country.
DIVINE CITIZENSHIP IS REVEALED THROUGH LIFE — “Brothers, join in imitating me, and keep your eyes on those who walk according to the example you have in us. For many, of whom I have often told you and now tell you even with tears, walk as enemies of the cross of Christ. Their end is destruction, their god is their belly, and they glory in their shame, with minds set on earthly things.”
Paul makes an impassioned plea for readers to study his life, and the life of those who follow his example. While you may think of this as an egocentric point of view, Paul calls for others to follow his example precisely because he is following Christ. Recall his challenge to the Corinthian Christians: “Be imitators of me, as I am of Christ” [1 CORINTHIANS 11:1 (quickview) ]. Christians are responsible to assess the manner in which life is lived for those who profess to be leaders. Leaders who are primarily concerned about enriching themselves reveal that they are “enemies of the cross of Christ.” Christians who “glory in their shame [because their] minds [are] set on earthly things” demonstrate that they are “enemies of the cross of Christ.”
This raises a point that must be stressed—Christians are called to a cruciform existence. This is nothing less than life marked by the cross as evidenced by suffering on behalf of Christ.  Paul has repeatedly pointed to the crucified life as his model. You will no doubt remember his powerful words written to the Christians of Galatia: “Through the law I died to the law, so that I might live to God. I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me” [GALATIANS 2:19 (quickview) , 20].
Earlier in this letter, Paul had written of his choice to live a life marked by sacrifice. “Even if I am to be poured out as a drink offering upon the sacrificial offering of your faith, I am glad and rejoice with you all. Likewise, you also should be glad and rejoice with me” [PHILIPPIANS 2:17 (quickview) , 28]. Likewise, he wrote of those sharing his missionary life as living a cruciform existence—Timothy [see PHILIPPIANS 2:19-24 (quickview) ] and Epaphroditus [see PHILIPPIANS 2:25-30 (quickview) ]. These examples demonstrate that for Paul, this was not a theoretical life—it was practical!