Does Paul point to any type of mystical experience when he speaks of "in Christ"? What does he express? It seems this phrase cannot be taken literally - so what is the deeper meaning of it, possibly from a philological point of view?
Since there are tons of different places in which Paul uses the phrase, some specific references might help here.
In Acts 24:24, for example, Paul refers to "faith in Christ Jesus." Faith here refers not to the pseudo-power claimed by the "word of faith" movement, but rather having faith in the testimony of Christ.
Romans 6:3, on the other hand, refers to being "baptized into Christ." Here the Greek word baptizo literally means "immersed" or fully a part of Christ. It could also refer, in this context, to being baptized in the name of Christ.
Romans 8:1 provides another variation of the phrase: "There is now no condemnation for those who are in Christ Jesus." [emphasis mine] Perhaps this is the type of phrase you're looking for, as Paul uses this terminology quite a bit. The Complete Jewish Bible may provide a clarification in its translation: "Therefore, there is no longer any condemnation awaiting those who are in union with the Messiah Yeshua." [emphasis mine] (Yeshua is the commonly accepted Hebrew name translated into Greek and Latin as Iesus, or into English as Jesus.)
Galatians 2:20, while it may not use the phrase in all translations, may further explain the concept. "I have been crucified with Christ, and it is no longer I who live, but Christ lives in me. So the life I now live in the body, I live because of the faithfulness of the Son of God, who loved me and gave himself for me." To Paul his old life as Saul of Tarsus, persecutor of Christians, was gone. Saul was dead, crucified as Christ was. His new life through a full-bonded union with Christ was represented by his new name, Paul the apostle. His life was completely subsumed by the lift of Christ. Obviously, as @Narnian points out, this is not a physical movement into a place or person, but a metaphysical or spiritual replacement.
To be in Christ is the safest place we can be. Outside of Christ--in other words, standing in our own merits--is the most dangerous place we can be. Paul tells us in 2 Corinthians
Notice that Paul uses the word in twice.
The first use of in tells us that our standing, or position, before God is not in our own merits, but in the merits of Christ. In other words, we cling to Him as our agent of salvation. In Christ, we are the righteousness of God (see 2 Corinthians 5:21). Our part is simply to believe; Christ did--and continues to do--what it took to reconcile us to God. Speaking more accurately, this reconciliation is of God to us.
The second in tells us that because of our sin, God had to turn His back to us. The good news is that God in Christ made it possible through Christ's cross-death for God to turn His face toward us. Judicially speaking, God then sees us in Christ. Again, this is the safest and best place in which anyone can be. Furthermore,
There will never come a time, either in this life or the next, when all true believers will not be in Christ. Scripture tells us that in the person of the Holy Spirit, Jesus is and always will be our advocate (Gk. paraklete) before the Father. From the NET Bible at Bible.org:
Hebrews 9:24 in the New Living Translation tells us
In conclusion, Christ's sacrificial death on our behalf will forever be our defense before a holy and righteous God. Christ, in effect, tells the Father whenever we sin,
When Satan, the accuser of the brethren (Revelation 12:10) accuses us before God, Christ comes to our defense. He is truly a lawyer in whom we can have full confidence! We will always be safe in His hands.
From a Word of Faith perspective as believers we experience a spiritual duality of location being both within our bodies and at one with Christ sharing together in an ether which is connected to Heaven and Earth.
This place is implicated through our total and complete union with He Who Was, Who Is and Who is to Come. This union grants us a place as a "cell" in the spiritual multicellular organism who is Christ the Almighty God. Unlike natural organisms the vitals of life depend and are drawn solely from the head who is God.
Our place within Christ may be severed should we or He choose as such. (Yes not all groups hold as such)
For further exposition please read the following:
And also a slightly different interpretation: