A.T. Robertson Commentary Ephesians 1:13

"in whom ye also, having heard the word of the truth, the gospel of your salvation,-- in whom, having also believed, ye were sealed with the Holy Spirit of promise," — Ephesians 1:13 (ASV)

Ye also (κα υμεις). Ye Gentiles (now Christians), in contrast to ημας (we) in 12.In whom (εν ω). Repeated third time (once in verse 11, twice in 13), and note ο or ος in 14.

Ye were sealed (εσφραγισθητε). First aorist passive indicative of σφραγιζω, old verb, to set a seal on one as a mark or stamp, sometimes the marks of ownership or of worship of deities like στιγματα (Galatians 6:17). Marked and authenticated as God's heritage as in 4:30. See 2 Corinthians 1:22 for the very use of the metaphor here applied to the Holy Spirit even with the word αρραβων (earnest).

Spirit (πνευματ). In the instrumental case.