Thielman’s discussion of the syntax is very helpful.
First adverbial clause: in whom you also, when you heard the word of truth, the gospel of your salvation,
Second adverbial clause: in whom you also, when you believed,
Main verb: were sealed….
We should probably not try to repair the broken syntax by supplying some verb in the first clause that could then transform it into a complex sentence with a pattern similar to the second clause. This is simply a broken sentence, or anacoluthon, common occurrence in the Pauline corpus.”Frank Thielman, Ephesians, BECNT (Baker, 2010), 78-79.
Both occurrences of ἐν ᾧ indicate union with Christ (the second in v. 13 being resumptive of the first) and being part of a series (see 1:7, 11, 13) (Campbell, 193-94; Larkin, 15; Hoehner, 235; Thielman, 79; Baugh, 96).
Most recent commentators do not think that the shift from 1st person to 2nd person between vv. 12 and 13 indicates a shift from Jewish Christians to Gentile Christians (Larkin, 15; Lincoln, 38; Hoehner, 235; Thielman, 78), and they are probably right. But Baugh’s contention that the shift highlights an emphasis in Gentile Christians being “full members of the covenant community in Christ,” anticipating ch. 2 is worth some further consideration (Baugh, 96-97). If I were to follow Baugh, I would understand the “we” to include both Jews and Gentiles and the “you” to focus on the Gentile Ephesians.
The Spirit is the seal, not the agent of the sealing (Lincoln, 40; Baugh, 97). Thielman is probably right that the “seal protects them from the wrath that God will one day pour out on the wicked” (Thielman, 77). It is not a reference to water baptism but to what water baptism signifies: Spirit baptism (Baugh, 98; Bock, 44).
Though I need to give more attention to the interpretation found in the KJV and Thomas Goodwin, namely, that the sealing followed believing (Goodwin, 1:237-38), the recent commentators make a cogent case that the hearing is “the hearing of faith” (Hoehner, 237-38) and that the hearing, believing, and sealing are contemporaneous (Lincoln, 39; Hoehner, 237-38; Thielman, 79; Baugh, 97; Bock 44).