There is an excellent answer already which essentially equates "In Christ" to refer to Christians generally. Another fine answer explains that the phrase other times does not equate to "Christian", but to other similar usages, such as God accomplishing salvation "In Christ" (Fuller's Christologically category).
I would go a step deeper and therein tie all these different usages together.
First, Strong's definition of "en":
a primary preposition denoting (fixed) position (in place, time or state), and (by implication) instrumentality (medially or constructively), i.e. a relation of rest (intermediate between (1519) and (1537))
Paul explains in Romans 6 that Christians have been baptized into Christ, and thus have both died with him, and now are "alive to God in Christ Jesus". This denotes a "rest" in Christ, or being positionally placed in Christ, in at least a spiritual sense.
[ESV] Romans 6:3-11
3 Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? 4 We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life. 5 For if we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his. 6 We know that our old self was crucified with him in order that the body of sin might be brought to nothing, so that we would no longer be enslaved to sin. 7 For one who has died has been set free from sin. 8 Now if we have died with Christ, we believe that we will also live with him. 9 We know that Christ, being raised from the dead, will never die again; death no longer has dominion over him. 10 For the death he died he died to sin, once for all, but the life he lives he lives to God. 11 So you also must consider yourselves dead to sin and alive to God in Christ Jesus.
Apologies for the long quote, but I believe it is the crux of what leads Paul to use the phrase "in Christ" (positional, temporal, resting in) so extensively. Paul states here that the very essence of being a Christian is a mystical/spiritual joining with Christ (Brenda Colijn's 5th Participatory category), stating that physical baptism is a symbol or type of this spiritual connection, through which Christians both die to sin (through Christ's substitutionary death on the cross) and live the Christian faith out to God (through being joined with him in his new life, post-resurrection).
This parallels Jesus teaching about him being in the Father, and the Father in him, and both of them living in a Christian.
[ESV] John 14:10-11
10 Do you not believe that I am in the Father and the Father is in me? The words that I say to you I do not speak on my own authority, but the Father who dwells in me does his works. 11 Believe me that I am in the Father and the Father is in me, or else believe on account of the works themselves.
[ESV] John 14:23
23 Jesus answered him, "If anyone loves me, he will keep my word, and my Father will love him, and we will come to him and make our home with him.
The scriptures make clear that Jesus is an example to all believers, who are also called disciples of Christ, and Jesus states clearly that he has no identity separate from the Father. In a similar fashion, therefore, Christians have no identity as Christians separate from being "in Christ" and, like Jesus, in the Father as well. This "in Christ" role of a Christian is further clarified in the same passage, with a discussion of Christ as the vine and Christians as the branches, where Jesus describe the relationship as one of "abiding".
[ESV] John 15:4-7
4 Abide in me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me. 5 I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing. 6 If anyone does not abide in me he is thrown away like a branch and withers; and the branches are gathered, thrown into the fire, and burned. 7 If you abide in me, and my words abide in you, ask whatever you wish, and it will be done for you.
Looking at the body of Paul's work, while there are different direct usages of "in Christ", they all come back to this joining with, or being enveloped by Christ's power/grace, the reliance believers have on Christ's atoning death to be their death, and Christ's new resurrection life to be their new spiritual life. It is through this connection of a believer in baptism into Christ that spiritually melds them with Christ - as Lord - and makes them a part of him.
Literally Paul describes the church (assembly of believers), as the body of Christ, with Christ as the head of that body.
[ESV] Ephesians 5
23 For the husband is the head of the wife even as Christ is the head of the church, his body, and is himself its Savior.
Thus Paul describes the life of a Christian as being bound up in Christ - a Christian's salvation as well as his power to live for God is bound up in the fact that he has given himself over to Christ's Lordship. Christ's essence becomes his essence. Christ's power becomes his power. If a Christian is not "in Christ", dead to his old self, and living a new life to God, then he is not really a Christian.
This is why Paul identifies a Christians by the reference to being "in Christ", and uses the phrase additionally in other similar contexts (i.e. God's work done for believer, benefits of Christians, etc.).