Being sealed by the Spirit is as sure as the indwelling of the Spirit.
2 Corinthians 1:21-22, Now he which stablisheth us with you in Christ, and hath anointed us, is God; Who hath also sealed us, and given the earnest of the Spirit in our hearts.
The very day a wretched sinner trust Jesus Christ to save them, they are not only indwelt by the Spirit of God but they are eternally secured by Him as well. It is not a sealing based on anything within the believer. It is totally based in the finished Work of Jesus on the Cross of Calvary.
Friend, if you are saved, not only does the Holy Ghost of God live inside you to guide, convict, instruct, grow and much more but He also seals you until the day of redemption.
What comfort that brings to the child of God when rough times in this life come!
