Paul has been speaking in the previous two verses about himself and the other Jewish believers, but now he begins speaking about the Ephesian believers.
Wax was placed on a letter and stamped with a symbol representing the person who wrote the letter. Paul uses this custom as a picture to show how God has used the Holy Spirit to assure us that we belong to him. Alternate translation: "God has sealed you with the Holy Spirit that he promised" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-activepassive]])