Having believed, you were marked in Him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance (Eph 1:13-14).
    

In addition to the spiritual blessings enumerated at the beginning of the 1st chapter, Paul says that believers were "marked" with a seal by the Holy Spirit, as a promise of what they will become and receive in the future.  This mark is the reverse of the "mark of Cain" (Gen 4:15), which was a sign of the curse for his sin of murder.  The Holy Spirit is a mark of blessing upon those who have been forgiven of sin.

         

It is interesting that the sign of an intangible but real inheritance is Himself invisible to fleshly eyes.  But if you can’t see a sign, what good is it?  In much of church history and theology, the Holy Spirit is represented as ethereal and mystical.  He is believed to be both omnipresent and hidden.  The usual substitute sign of identification of Christian faith is subscription to a particular doctrinal statement.  This easily allows an outsider to classify a person as a believer, and it is also a "guarantee of inheritance" to the believer himself:  "Because I have confessed Christ publicly by word and baptism, I know that I am His for eternity."
        
Paul doesn’t go in this direction, though.  Rather, it is not our confession that seals us, but God’s act, God's attestation.  And for Paul, it is something more concrete than traditional fuzzy depictions of the role of the Holy Spirit. The believer "knows" he has been sealed, there is some form of inner witness, or something more.   Perhaps Paul recalls his own first encounter with the Holy Spirit when he ran into a divine wall on the road to Damascus:  Wham!, falling to the ground, pain, blindness, a voice, and darkness that lasted three days.  At the end of this traumatic time came healing, baptism, breaking his fast, and proclaiming the Messiahship of Jesus (Acts 9).   Who was it who said, "The Holy Spirit is a gentleman" -- ?  Not Paul!

         

This opens up to us a further understanding of Pentecost: both the historical event and the personal experience.  We see that it is not only to "receive power" (Acts 1:8) for service, for mission, for martyrdom.  It is also to receive a sign of election, a mark that is visible to astounded and scandalized onlookers, as well as palpable to the recipients.  A denominational Pentecostal would point to speaking in other tongues as the definitive sign, since we know that this phenomenon occurred both to the disciples at Pentecost, to Cornelius at Caesaria, and was a later regular practice of Paul himself (1 Cor 14:18).  But Ephesians doesn’t identify tongues with the seal.  

          

What we can say is that the first Pentecost opened up and established a greater Presence of God in the disciples' lives, an ongoing distinction from those around them.  And the purpose of God was to make that a perpetual endowment for all members of His church:

​

          "Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins.  And you will receive the gift of the Holy Spirit.  The promise is for you and your children and for all who are far off--for all whom the Lord our God will call" (Acts 2:38-39).

​

           This is from Peter’s sermon on the Day of Pentecost, yet it contains the same note of predestination as Paul's message in Ephesians.  God calls the believers to receive this gift which both seals them and distinguishes them from non-Christians.  It is not the same as their confession of faith, but it is an act of God in the believer subsequent to that confession that establishes their identity and gives them an assurance for eternity.