Because, as believers, we are sealed with the Holy Spirit of promise (Ephesians 1:13) we are assured of the reality of the salvation that has been won for us by the Lord Jesus Christ. The importance of the Holy Spirit’s indwelling presence is to insure that we understand that our salvation is complete and sure, and not based on our own “good” works, but only on the work of Christ.
The sealing of the Holy Spirit, not only enables us to be secure in our relationship with Christ, but announces to our enemy Satan, that whether he likes it or not, we belong to God, and are sent forth into this world with God’s authority to proclaim the truth, and through that truth transform others into the Image of God.
As one who has committed their lives to the truth of the gospel, we do not need to fear that we will somehow be lost in the transition, or that we will do, think or say something that will forfeit our salvation. We are secure in our relationship with the Lord Jesus Christ. That should give us comfort in times of difficulties, and will remind us that He is always with us.