In Christ we have also obtained an inheritance, having been destined according to the purpose of him who accomplishes all things according to his counsel and will, so that we, who were the first to set our hope on Christ, might live for the praise of his glory. In him you also, when you had heard the word of truth, the gospel of your salvation, and had believed in him, were marked with the seal of the promised Holy Spirit.
(New Revised Standard Version)
I have some impressive documents accompanied by seals and red ribbons from a number of Bishops who appointed me to various jobs over the years. The idea of a seal has almost gone out of use in ordinary life, although not I think in the legal one. The impress of a signature pressed into hot wax, was one way in the past, to ensure that the Bishop or King was authorising something - in my case as Vicar or Rural Dean, or Diocesan Officer.
Our reading today speaks of us being 'marked with the seal of the promised Holy Spirit.' At our baptism we were washed clean of our sins, and sealed, if not with a ring and hot wax, then with a cross drawn across our forehead. This seal indicates ownership, just as in ancient times a payment was made on goods and a seal applied by the owner, so he would know his goods when he came to collect them.
God has paid a down-payment on us and he has a given us His Holy Spirit as members of His heavenly family and a guarantee of what we will one day inherit if we continue along the path that He has set before us.
Lord God Almighty,
You have made us Yours,
through Your Holy Spirit.
You have marked us as Your people
through the cross of Your son
our Saviour, Jesus Christ.
Make us worthy of our calling
and keep us steadfast in the faith.