1 Cor. 1:30: “But it is due to Him that you are in Christ Jesus, who became to us wisdom from God, and righteousness and sanctification, and redemption.”
Rom. 8:1: “Therefore there is now no condemnation at all for those who are in Christ Jesus.”
2 Cor. 5:17, 21: “Therefore if anyone is in Christ, this person is a new creation; the old things passed away; behold, new things have come. 21 He made Him who knew no sin to be sin in our behalf, so that we might become the righteousness of God in Him.”
Eph. 1:4: “just as He chose us in Him before the foundation of the world, that we would be holy and blameless before Him.”
The ἐν (“in”) + the dative construction was a favorite linguistic formula of the Apostle Paul. For example (NA28th), ἐν αὐτῷ (“in Him”) appears 23 times and ἐν Χριστῷ (“in Christ”) appears 73 times in his literature.
As with all prepositions, the meaning of preposition ἐν is determined by its object, which in Paul’s formula is either singular dative nouns such as Ἀδὰμ (“Adam”), σαρκὶ (“flesh”), νόμῳ (“law”), πνεύματι (“Spirit”), Κυρίῳ (“Lord”), or the singular dative pronoun, αὐτῷ (“Him”). Note that when Christ is the object of ἐν (“in Him,” “in Christ,” “in the Lord,” etc.), a few views have been suggested among standard scholarship as to the semantic of the dative in the prepositional phrase.
For example, dative of sphere (i.e., locative, “in the realm/sphere of”); instrumental dative (i.e., dative of means/instrument, “by means [instrumentally] of”), and a couple other semantic views have also been posited. Wallace (GGBB) points out, that the dative expressing sphere is a frequent usage especially with ἐν + the dative, as in Eph. 1:4, “Just as He chose us in Him [ἐν Κυρίῳ] before the foundation of the world, that we would be holy and blameless before Him. In love.”
However, it seems that the predominant meaning (i.e., of scholarly opinion, e.g., Lightfoot, TDNT, NIGTC et al.) of Paul’s ἐν + the dative formula, when referring to Christ (or other persons in the Trinity) denotes “in union with Christ,” that is, being identified and unified with Him in His life, death, and resurrection. Consistent to Paul’s Adam-Christ antithesis, which is well exampled in such passages as 1 Cor. 15:22, where Paul’s ἐν + the dative formula denotes “in union with”: “For as in [union with] ADAM [ἐν τῷ Ἀδὰμ] all die, so also in [union with] CHRIST [ἐν τῷ Χριστῷ] all will be made alive.” Turner sees that “Adam is a representative man ‘in’ whom all mankind was viewed. . . . But the instances with en are predicated of Christ or the Gospel and mean ‘in the sphere of’” (cf. Matt. 3:11; Gal. 3:27; 1 Cor. 12:13).
Many other passages in Paul’s literature, which involve the ἐν + the dative formula, denote union or identification with Christ in contrast to condemnation, sin, law, flesh, etc. For example, Romans 6:11: “Even so consider yourselves to be dead to sin, but alive to God IN CHRIST [ἐν Χριστῷ]; 8:1: “Therefore there is now no condemnation for those who are IN CHRIST [ἐν Χριστῷ] Jesus.”
In this sense, men walk to two territories, either “in Christ” or “in Adam, in the flesh.” Union and identification with one or the other. Being “in Christ,” therefore, is tantamount to being in union with Him. So Lightfoot observes, “ἐν Χριστῷ, i.e. ‘by virtue of our incorporation in, our union with, Christ.’” It is in that sense, then, that as the old man is ἐν ‘Αδάμ and the Jew ἐν νόμῳ, the believer are ἐν Χριστῷ. So Eadie rightly says, “Believers were looked upon as being in Christ their federal Head, when they were elected.”
Further, note the contrast of being ἐν σαρκὶ (“in [the] flesh”) and being “in Christ/“in the Spirit” with emphasis on the indwelling of the Spirit in Rom. 8:9: “However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him.” Thus, being ἐν Χριστῷ is set in contrast to being ἐν ‘Αδάμ (“in Adam”) in 1 Cor. 15:22 and being found ἐν Χριστῷ is set in contrast to being ἐν νόμῳ (“in the Law”) in Phil. 3:6-9.
The ἐν + the dative formula (esp. Paul’s near exclusive use of ἐν Χριστῷ) undeniably expresses the love that God the Father has for believers, which is shown by His electing them to the adoption of sons placing them in union with His Son (Eph. 1:4-5). Paul’s ἐν + the dative formula linguistically denotes “in union with Christ,” that is, being identified and unified with Him in His life, death, and resurrection. Hence, Paul can say, “Therefore I, the prisoner ἐν κυρίῳ [‘in the Lord,’ not, ‘of the Lord’]” (Eph. 4:1). Paul also uses this signifying formula to denote the unbreakable union believers have with the Father and the Spirit—viz. union and identification with the triune God.
 Aside for three places in Peter, ἐν Χριστῷ appears only in Paul.
 Ἐν αὐτῷ, ἐν Χριστῷ, ἐν Κυρίῳ.