In Him you also trusted, after you heard the word of truth, the gospel of you salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of purchased possession, to the praise of His glory. Ephesians 1:13-14 (NKJV)
I think it is important to look back on our life before we knew Christ when we had no hope and were heading down the road to destruction that would have ended in death. This is why the verses about the gospel and living for eternity is so close to my heart. Living without God, Jesus Christ, and the Holy Spirit it is impossible to reach out eternal destiny. God knew this all along and had a plan for us.
This is God's magnificent plan for us once we accepted Christ we were redeemed by the blood of Christ, forgiven, and adopted (being called children of God), and sealed with the Holy Spirit for God's own possession. We were chosen and predestined for this before the foundation of the world, determined from eternity, brought to pass through the exercise of God's will.
God is so loving and compassionately committed to having us for His own possession always dedicating His life to save the sinner. This is why He sent His Son to die for us so we would be saved by His amazing grace. This is why we are sealed with the Holy Spirit as a guarantee for our inheritance knowing we are secured forever in His love and power.
The Holy Spirit is a preserving seal. He locks in our love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, and self-control. Once something is locked in from the Holy Spirit it will always remain in us. When something is sealed and being locked in this also locks out doubt and unbelief. The Holy Spirit encourages us to believe in the promises of God. Believing in God's word magnifies His glory.
The Holy Spirit is an authenticating seal. The Spirit works in our lives transforming us into the children of God that lives according to the Spirit and according to God's will. He is the sign of divine reality in our lives. The Holy Spirit validates us as being genuine, valuable, and having the seal of approval as we enter into heaven.
The Holy Spirit is a protective seal. The Holy Spirit protects us from destructive forces. We are protected from satan, to whom the seal identifies us as off limits he can attack, but since we are marked as God's possession he can't overtake us and make us his own once we are called children of God.
The Holy Spirit is a seal guaranteeing our future redemption. Though believers have not yet experienced redemption in full, God will bring about final redemption as we enter into heaven, living life in His immediate presence, for all who have received the Holy Spirit. We will receive our inheritance which is promised to us and praise God forevermore glorifying His Holy name.