The phrase "in Christ" appears over 160 times in Paul's letters. It is not a pious add-on. It is the most compressed description of a Christian's fundamental identity. You are not a person who happens to follow Christ. You are, in Paul's framework, a person whose existence is now located inside Christ — his death is your death, his resurrection is your resurrection, his standing before the Father is the standing in which you stand.
The Roman adoption background to Romans 8:15 and Galatians 4:5 is important for understanding what Paul's first readers heard. Roman adoption was not a sentimental act — it was a full legal transfer. The adoptee's past creditors had no further claim. His previous legal obligations were cancelled. He received the full rights of a biological son, including inheritance. The father's status became his status. This is what huiothesia — the Greek word translated "adoption" — meant in the Roman world. Paul chose this word deliberately for a Roman audience.
2 Corinthians 5:17 — "Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new" — the word translated "creature" is ktisis, which Paul elsewhere uses for the whole created order. You are not an old person with improvements. You are a new kind of thing that did not previously exist in the history of the cosmos. The old identity framework — built from family, tribe, achievement, failure, reputation — has passed away. This is not an instruction to feel differently. It is a declaration about what is objectively true.
The Ephesians 2:6 claim is among the most startling in the New Testament: God "hath raised us up together, and made us sit together in heavenly places in Christ Jesus." Seated. Past tense. Not "will seat you when you improve." The seating has happened. This is the exalted position of the ascended Christ, and Paul is saying you already occupy that position in him. The identity crisis that drives so much searching is answered not by discovering something about yourself but by receiving a declaration about where you already are.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.