What is the seal? It is not ‘what’. It is not a thing; it is a person; it is the Holy Spirit: he is the seal.
Once you were his, and you gave your life to him and repented of your sin and came, you were ‘sealed with that holy Spirit of promise.’ Now the seal is the seal put on a document to attest that it is sent from a particular person. It is a legal document perhaps. The seal attests the validity of the signatory. What happened to you in conversion? You were sealed; it was an authenticating sign. Certain things have taken place. Your understanding has been opened, your life has been changed, your desires and hopes have been transformed. Now you are concerned to live for Christ, now you have an active conscience and you want to obey him and follow him. These things are clearly the work of the Spirit.
You have changed. All the things that you have loved to do that were sinful and worldly, you cannot do them anymore. Now you want spiritual things and Christian companionship and the word of God, and you are in the family and you love God. These changes are authenticating signs that you are an authentic Christian. You bear the seal that he has worked in your heart. You were sealed with that Holy Spirit of promise. It is God’s seal of ownership upon you. It is God’s assurance to you that you are saved, now and for ever.