In Paul’s time and context this word (sealed) conveyed several meanings, including protection and preservation (ESV notes, O’Brien), certifying our authenticity (ESV – ‘royal seal’), or a mark of ownership (Stott, O’Brien). The Holy Spirit indwells believers in Christ (Rom 8:11; 1 Cor 3:16) and is the proof that we are genuine believers; that we belong to God. He also is the seal of protection (Rev 7:3; 9:4). God has set his seal on his chosen people, who believe on Christ at the preaching of the gospel!
Now Paul refers to the Holy Spirit as the one who was ‘promised.’ Though saints in the Old Testament were brought to life by God’s Spirit, and God’s Spirit was active in their lives as the people of God, only prophets, priests, and kings experienced the Spirit in great measure. But the promise was given by God that in the New Covenant, the Holy Spirit would be given in a greater way to all God’s people!
And it shall come to pass afterward, that I will pour out my Spirit on all flesh; your sons and your daughters shall prophesy, your old men shall dream dreams, and your young men shall see visions. Even on the male and female servants in those days I will pour out my Spirit. Joel 2:28-29, ESV (see also Ezekiel 36:27)Jesus also gave this promise:
On the last day of the feast, the great day, Jesus stood up and cried out, “If anyone thirsts, let him come to me and drink. Whoever believes in me, as the Scripture has said, ‘Out of his heart will flow rivers of living water.’” Now this he said about the Spirit, whom those who believed in him were to receive, for as yet the Spirit had not been given, because Jesus was not yet glorified. John 7:37-39, ESV (see also John 14-16; Acts 1:4-5)True to his promise, the Lord poured out his Spirit on his people on the day of Pentecost (Acts 2:1-4).
Christ redeemed us from the curse of the law by becoming a curse for us—for it is written, “Cursed is everyone who is hanged on a tree”—  so that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we might receive the promised Spirit through faith. Galatians 3:13-14, ESVThe Guarantee. “who is the guarantee of our inheritance…” (Eph 1:14a). It keeps getting better! Not only do we have the Holy Spirit of promise as our seal, declaring that we are chosen and loved and owned by God, but now Paul reveals that the Spirit is the ‘guarantee of our inheritance!’
...and who has also put his seal on us and given us his Spirit in our hearts as a guarantee. 2 Corinthians 1:22The indwelling Spirit of God is our guarantee...for how long? “...until we acquire possession of it…” (Eph 1:14b). This phrase, like the one in verse 11, is either speaking of our obtaining an inheritance from God, OR God choosing us as his own inheritance (both are biblically true). It is either saying that the Spirit is our guarantee until we acquire the rest of our inheritance, OR that he is our guarantee until “God redeems his possession” (ESV notes). This is consistent with how God refers to his people in other places:
They shall be mine, says the LORD of hosts, in the day when I make up my treasured possession… Malachi 3:17, ESV (see Exodus 19:5; Deut 14:2; Peter uses this language of the church in 1 Peter 2:9 – O’Brien)Again, either way, the meaning is clear. God has given us the Holy Spirit, guaranteeing to finish the salvation work he has begun until the day we are together with him in glory! We will be his treasured people, and he will forever be our treasure!
…set your hope fully on the grace that will be brought to you at the revelation of Jesus Christ. 1 Peter 1:13, ESVJohn Piper speaks of the encouragement this gives us: "We are secure in God’s love and power. Warnings in Scripture are written to threaten our security in everything but God. Sin is an effort to feel secure in anything other than God. …God seals the believer with the Holy Spirit and guarantees that we will come to our inheritance praising his glory."
- The Letter To The Ephesians; Peter T. O’Brien; ©1999; Wm. B. Eerdmans Publishing Co. Grand Rapids, MI.
- The Message Of Ephesians – God’s New Society; John R. W. Stott; ©1979; InterVarsity Press, USA. Downers Grove, IL.
- Sermon by John Piper: Sealed By The Spirit To The Day Of Redemption
- ESV Study Bible Notes; ©2008 Crossway Bibles; Wheaton, IL.