The Holy Spirit is the prominent promise of the Old Testament (see Is 32:15, 44:3, Ez 36:27, 39:29, Joel 2:28, Zech 12:10). He is also the promise which Christ left with His disciples (Lk 24:49, Jn 14:16-17, 16:13-14, Gal 3:14). It was not until Jesus was glorified that the promise of the Holy Spirit was realized (Jn 7:30, Acts 1:4-5, 19:2). It is no wonder that Paul refers to the Divine Person here as, ‘the Holy Spirit of promise.’ He is both the gift of the New Covenant, and the guarantee of our future inheritance. He is likened to the engagement ring given upon the promise of marriage by the bridegroom.
Sealing is a manifold work – the word sfragidzo refers to a stamp of authority, as a king would seal a letter with his signet ring impressed upon a wax seal. An unbroken seal is a token of authority, ownership and safety. As such we can imagine the Holy Spirit as God’s signet ring, impressing the image of His Son upon the human heart, marking it out as His own (2 Tim 2:19), and indicating that whatever is marked out by him will safely return home to Him (see also Rev 7:3, 9:4). The word is not often used in the New Testament; at times it refers to a literal seal, as at the tomb of Jesus Christ (Mt 27:66). It is notable that when the word appears as an illustration of that which is spiritual, it is usually linked to both the Holy Spirit and the act of believing on the part of the individual (see Jn 3:32-34, 6:27-29, 2 Cor 1:18-22).
