The Holy Spirit is God’s first installment on your eternal inheritance.
The Holy Spirit’s ministry in your life is multifaceted and profound. Among other things He brings salvation, conviction, guidance, and strength. He indwells and equips you for spiritual service and gives assurance of your salvation. He is your Helper and Advocate. He is the Spirit of promise, who seals you until the day when your redemption is fully realized (Eph. 4:30).
Sealing speaks of security, authenticity, ownership, and authority. Ancient kings, princes, and nobles placed their official seal on documents or other items to guarantee their inviolability. To break the seal was to incur the wrath of the sovereign whom it represented (cf. Dan. 6:17; Matt. 27:62-66).
A seal on a letter authenticated it as from the hand of the one whose seal it bore. Legal documents such as property deeds and wills were often finalized with an official seal. Those who possessed the sealed decree of a king had the king’s delegated authority to act on that decree.
Each of those aspects of sealing is a picture of the Spirit’s ministry. He is God’s guarantee that your salvation is inviolable and that you are an authentic member of His kingdom and family. You are His possession—having been purchased with His Son’s precious blood (1 Cor. 6:20). You are His ambassador with delegated authority to proclaim His message to a lost world (2 Cor. 5:20).
The Spirit is the pledge of your eternal inheritance (Eph. 1:14). The Greek word translated “pledge” in that verse (arrabōn) was used of down payment or earnest money given to secure a purchase. Rejoice in the assurance that God, who cannot lie (Titus 1:2), has given you His Spirit as a guarantee that He will keep His promises.
From “Drawing Near” by John MacArthur.