Sealed by the Holy Spirit.

...you were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory. Ephesians 1:14

A seal is a device for making an impression in wax, clay, paper, or some other medium, including an embossment on paper, and is also the impression thus made. The original purpose was to authenticate a document, a wrapper for one such as a modern envelope, or the cover of a container or package holding valuables or other objects.

Because seals are commissioned by individuals and carved by artists, every seal is unique, and engravers often personalize the seals that they create. The materials of seals and the styles of the engraving are typically matched to the personalities of the owners.

 http://en.wikipedia.org/wiki/Seal_(emblem)

A photograph of a wax seal. The wax seal shows the Brodtkorb family coat of arms, 

by André R. Brodtkorb

Seals bear the name of families, like the one above, or a business or government for whom they are made.

The seal of God is the Holy Spirit. As a seal leaves an impression on wax or paper, so the Holy Spirit impresses God's image on us. God's name is on us - He has sealed us and called us his own. I love this image. Here are some more verses on our being sealed.

And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption. Let all bitterness and wrath and anger and clamor and slander be put away from you, along with all malice.Be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you.Therefore be imitators of God, as beloved children. And walk in love, as Christ loved us and gave himself up for us, a fragrant offering and sacrifice to God.  Ephesians 4:30

And it is God who establishes us with you in Christ, and has anointed us, and who has also put his seal on us and given us his Spirit in our hearts as a guarantee (a pledge). 2 Corinthians 1:21,22

But God’s firm foundation stands, bearing this seal: “The Lord knows those who are his,” and, “Let everyone who names the name of the Lord depart from iniquity.” 2 Timothy 2:19

Then I saw another angel ascending from the rising of the sun, with the seal of the living God, and he called with a loud voice to the four angels who had been given power to harm earth and sea, saying, "Do not harm the earth or the sea or the trees until we have sealed the bond-servants of our God on their foreheads." Revelation 7:2,3