From Walter Liefeld’s commentary on Ephesians
The Holy Spirit is involved in two ways to make that possession — both God’s and ours — secure.
First, he is God’s seal on us. The reference is to the ancient practice of identifying and reserving an item, in particular a scrolled letter, for its proser designee. We belong to God, and he has marked us for himself (from BDAG: to mark something with a seal as a means of identification, so that the mark denoting ownership also carries with it the protection of the owner. This forms the basis for understanding the imagery which speaks of those who enter the Christian community as being sealed with or by the Holy Spirit). The seal is the Holy Spirit himself, who indwells the believer. Since there is no grammatical indication that a lapse of time occurs between “believing” and being “sealed” by the Spirit, the soundest assumption is that the use of the aorist tense for “having believed” signifies the same time as the sealing.
Second, the role of the Holy Spirit is also to function as a deposit (today, we might say “down payment”) or as a pledge (v. 14). God will complete the purchase, so to speak, when he redeems his possession, the believer. It is striking that Paul describes the work of the Spirit in commercial terms. While a comparison of the Holy Spirit with a deposit might seem unusual, it drives home the point that God has paid mightily for his possession and will not give it up. While we sometimes (understandably) ponder whether we might lose our salvation, God assures us that he will not lose his possession.