in whom you also, having heard the word of the truth, the Good News of your salvation—in whom, having also believed, you were sealed with the Holy Spirit of promise
in whom you also - By saying "you also", Paul includes Gentiles with himself (a Jew).
- Ephesians 1:13
having heard the word of the truth - Paul says this in response to accusations that he was lying about his message of salvation.
the Good News of your salvation - Paul applies "The Good News" to Gentiles as well as Jews.
in whom, having also believed - This is in contrast to the false teaching that one could achieve salvation through works, according to the Way of the Flesh, and faith wasn't needed.
you were sealed with the Holy Spirit of promise - The Holy Spirit has a role in two different inheritances. One is our inheritance. See Eph. 1:14 for the Holy Spirit's role in our inheritance. The other inheritance is God's inheritance. In ancient times, when something was purchased, a deed of purchase was kept sealed as proof of ownership. An official seal was marked in clay in order to seal the document. Those who believe in Christ are sealed with the Holy Spirit. It is the Holy Spirit that is the identification mark that shows that we are God's possession. Paul is saying that not only are the Jews God's inheritance but all those who believe in Christ are as well.