Here is Thayer's Greek Lexicon definition of sphragizō as used in Ephesians 1:13:
τινα τῷ πνεύματι and ἐν τῷ πνεύματι, respecting God, who by the gift of the Holy Spirit indicates who are his, passive, Ephesians 1:13
Below is from ISBE:
God by His Spirit indicates who are His, as the owner...