What does it mean to be "in Christ"?

For 2017, I chose the phrase "in Christ" to focus on.  I am about to wrap up this look at union with Christ, but I have a few more weeks to go to finish in 2018. Each week, I will post verse(s) that includes the phrase "in Christ". I look forward to growing in understanding of what it means to be "in Christ."

⁸ Accordingly, though I am bold enough in Christ to command you to do what is required, ⁹ yet for love's sake I prefer to appeal to you—I, Paul, an old man and now a prisoner also for Christ Jesus— ¹⁰ I appeal to you for my child, Onesimus, whose father I became in my imprisonment. ¹¹ (Formerly he was useless to you, but now he is indeed useful to you and to me.) ¹² I am sending him back to you, sending my very heart. ¹³ I would have been glad to keep him with me, in order that he might serve me on your behalf during my imprisonment for the gospel, ¹⁴ but I preferred to do nothing without your consent in order that your goodness might not be by compulsion but of your own accord. ¹⁵ For this perhaps is why he was parted from you for a while, that you might have him back forever, ¹⁶ no longer as a bondservant  but more than a bondservant, as a beloved brother—especially to me, but how much more to you, both in the flesh and in the Lord.
¹⁷ So if you consider me your partner, receive him as you would receive me. ¹⁸ If he has wronged you at all, or owes you anything, charge that to my account. ¹⁹ I, Paul, write this with my own hand: I will repay it—to say nothing of your owing me even your own self. ²⁰ Yes, brother, I want some benefit from you in the Lord. Refresh my heart in Christ. (Philemon 1:8-20)