The Holy Spirit is described as a seal three times by Paul in the New Testament (2 Cor. 1:22; Eph. 1:13-14; 4:30). What did the apostle have in mind by this illustration?
A seal was used to indicate ownership. For example, a seal would be used by a merchant to indicate he had purchased a product. In addition, a seal would be used to show that an item belonged to a wealthy person. Further, a seal could be used to brand slaves or animals. And a seal could be put on a document to indicate authenticity.
The reference in Ephesians 1 is found in a passage in which Paul is describing the spiritual blessings that every believer has. Paul highlights that the blessings are connected to the salvation provided by the Trinity. He says that the Father seals each of his people with the Holy Spirit as one of the consequences of their union with Christ. The sign that they are owned by God, that they are genuine believers, is the presence of the Spirit. He is given to them at the moment of conversion.
In contrast to the examples of seals mentioned above, which were only signs, the seal that marks every Christian is alive. So the giving of the Spirit is connected to the ongoing supply of spiritual life. Moreover, the Giver of this seal has great love for the ones he sealed. In providing the Spirit for each believer, the Father was displaying and confirming his love for that Christian. And since the seal will not be removed from the believer, the giving of the Spirit is a reminder that this relationship is permanent (unlike products purchased by a merchant, believers will not be sold on to another person).
Those aspects of life, love and permanence are important to remember when we read Paul's reference in Ephesians 4:30 concerning grieving the Holy Spirit. Christians grieve him when they sin. This description of the Spirit's response is a reminder that he has emotions, and that he does not respond to our sins with indifference. Yet the sins of believers do not mean that they cease to have life, or that they are no longer the objects of divine love, or that God wishes to bring his relationship with them to an end. They are sealed with the Spirit.