In 1970, Stevie Wonder released Signed, Sealed, Delivered, which became one of his biggest hits and part of our cultural fabric and collective memory. For most of us, when we see that title, our minds play the music and lyrics. Go ahead. Take a moment. I’ll wait on the soundtrack in your head. 
What does it meaned to be sealed? Well, first off, divest yourselves of the image of Tupperware being "burped" for freshness. Rather, consider the envelope in the image accompanying this post.
When an envelope is sealed, its contents are meant to be kept and protected. And that is what the Holy Spirit does for us. When we are sealed with the Spirit, we are kept and protected until the day of our redemption, which is our ultimate salvation from sin and death and being brought into eternal life and communion with God. In the words of another song, When I Get Where I'm Goin' by Brad Paisley, "I will shed the sins and struggles I have carried all these years.... And [I'll] see my Maker's face. I'll stand forever in the light of His amazing grace."
2 Corinthians 1:21-22 tells us that, if we are in Christ, then God seals us and places His Spirit in our hearts as a guarantee (i.e., a pledge similar to a down payment or security deposit for the future fulfillment of a promise).
But as a guarantee of what? you might ask. A guarantee of the eternal life mentioned in John 3:16: “For God so loved the world that He gave His only begotten Son, that whosoever believes in Him should not perish but have everlasting life.”
Ephesians 1:13-14 reiterates this, saying that, after we heard the truth of the gospel and trusted in Christ, we “were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance.” What is our inheritance? Again, it is eternal life.
In other words, when we place our faith in the finished work of Jesus on the cross as payment for our sins, we become saved individuals by the grace (i.e., unmerited favor) of God – “For by grace you have been saved through faith, and that not of yourselves; it is the gift of God.” (Ephesians 2:9) And at the moment of our salvation, God’s Holy Spirit indwells us, assuring us that we are His children and guaranteeing that He will make good on His promises of life everlasting and eternal security.
Many years ago, I read Charles Stanley’s book Eternal Security, which asked the question, “Can you be sure?” Many saved individuals struggle with the assurance of their salvation and lack certainty about going to heaven and having eternal life. In his book, Rev. Stanley writes about his father-in-law, who, although he had accepted Christ as his Savior, could never come to a full assurance of his eternity.
But even if someone struggles, it doesn’t change the reality of eternity. The Bible teaches it, not as supposition but as fact. I imagine that many in that camp are pleasantly surprised and relieved when they enter the hereafter and find that everything God promised us in His Word is completely true. It would have been more pleasant for them to have experienced that assurance in this lifetime, but the ultimate fullfillment of it is nonetheless a great blessing.
Others believe it is possible to lose one’s salvation, a concept erroneously derived from Galatians 5:6, which discusses “falling from grace.” But that is taken out of context. In that particular passage and its adjoining verses, the Apostle Paul makes a distinction between the old covenant of law that God established with the children of Israel, and the new covenant of grace that Jesus brought. Paul told believers that circumcision (a symbol of the old covenant) was no longer a requirement and that it no longer held any spiritual importance.
Paul encouraged followers of Jesus to be steadfast in the liberty that the new covenant had brought and not to put themselves back into the bondage of having to fulfill the laws and rituals of the old covenant. He told them that, if they clung to the law as the basis of their salvation rather than the work of Christ on the cross, they were estranging themselves from Christ and falling from (i.e., choosing to reject) grace.
While some would debate it, I steadfastly believe in the concept of “once saved, always saved,” and I believe it is biblically sound doctrine. In terms of losing one’s salvation, I did nothing to save myself, so there is nothing I can do to unsave myself. And since God has imparted His Holy Spirit to me as a guarantee of my salvation and eternal destiny, He’s not going to unsave me either. God would not be God if He broke His promises.
And if we continue with Stevie Wonder’s lyrics, we find that what follows “signed, sealed, delivered,” are the words “I’m yours! You got my future in your hands.”
Never have truer words been written, sung, or spoken that sum up our security in Jesus Christ. We are His, and He has our future in His hands!
I pray this brings comfort and peace to those who read it, and if we struggle with doubt and uncertainty, let us speak the truth of God’s Word to our hearts. As Jesus said in John 8:31-32, if we abide in His Word, we will know the truth, and the truth will set us free.