Paul was speaking in the previous two verses about himself and the other Jewish believers, but now he begins speaking about the Ephesian believers.
“Word” here is a metonym for “message.” AT: “God’s message about truth” (See: rc://en/ta/man/translate/figs-metonymy)
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. AT: “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)
Receiving what God has promised is spoken of as though one inherits property or wealth from a family member. AT: “the guarantee that we will receive what God has promised” (See: rc://en/ta/man/translate/figs-metaphor)