To live is Christ

Early in his letter to the Philippians, Paul encourages them not to worry about his imprisonment in Rome (and all that led up to that) because he can see the man behind the curtain, so to speak. He can see the Hand of Providence moving through his circumstances so that the gospel is being advanced in them. And he can therefore rejoice in them. Christ is being magnified in that advance and will be honored in what comes next--whether it's his execution or it's his release. Paul will rejoice in either of those, because Christ will be honored in his life or in his death. Why? Well for starters, to him, "to live is Christ."
As I was studying this recently, I was struck by the words of John Eadie,

Might not the sentiment be thus expanded? For me to live is Christ--the preaching of Christ is the business of my life; the presence of Christ the cheer of my life; the image of Christ the crown of my life; the spirit of Christ the life of my life; the love of Christ the power of my life; the will of Christ the law of my life; and the glory of Christ the end of my life. Christ was the absorbing element of his life. If he traveled, it was on Christ's errand; if he suffered, it was in Christ's service. When he spoke, his theme was Christ; and when he wrote, Christ filled his letters.

Hendrickson isn't as poetic, but it does leave us a trail through Paul's thought, (just for ease of use, I threw his proofs down below so you can just scroll down to look at them).

to derive one's strength from Christ (Phil. 4:13), to have the mind, the humble disposition of Christ (Phil. 2:5-11), to know Christ with the knowledge of Christian experience (Phil. 3:8), to be covered by Christ's righteousness (Phil. 3:9), to rejoice in Christ (Phil. 3:1, 4:4), to live for Christ, that is his glory (2 Cor. 5:15), to rest one's faith on Christ and to love him in return for his love (Gal 2:20).

And as wonderful as all that is, how that union, that identity is something we all would desire, long for...we need to remember the end of Paul's line: and to die is gain. It only gets better brothers and sisters. We can't lose sight of that.
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Phil. 4:13: I can do all things through him who strengthens me.
Phil. 2:5-11: Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but made himself nothing, taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross. Therefore God has highly exalted him and bestowed on him the name that is above every name, so that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.
Phil. 3:8: Indeed, I count everything as loss because of the surpassing worth of knowing Christ Jesus my Lord. For his sake I have suffered the loss of all things and count them as rubbish, in order that I may gain Christ
Phil. 3:1: Finally, my brothers,[a] rejoice in the Lord. To write the same things to you is no trouble to me and is safe for you.
Phil. 4:4: Rejoice in the Lord always; again I will say, Rejoice.
2 Cor. 5:15: and he died for all, that those who live might no longer live for themselves but for him who for their sake died and was raised.
Gal. 2:20: 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.