Ephesians 1:13-14(The Message)
13-14It's in Christ that you, once you heard the truth and believed it (this Message of your salvation), found yourselves home free—signed, sealed, and delivered by the Holy Spirit. This signet from God is the first installment on what's coming, a reminder that we'll get everything God has planned for us, a praising and glorious life.
Ephesians 1:13-14(New International Reader's Version)
13 You also became believers in Christ. That happened when you heard the message of truth. It was the good news about how you could be saved. When you believed, he marked you with a seal. The seal is the Holy Spirit that he promised.
14 The Spirit marks us as God's own. We can now be sure that someday we will receive all that God has promised. That will happen after God sets all of his people completely free. All of those things will bring praise to his glory.
Since ancient times, seals have been used to autheticate documents and ensure against forgery, as well as to display the power of a monarch or kingdom. The seals of kings were usually born on a ring, then impressed in wax, and it signified the power of the bearer. The seal could mean that you were under protection, or it could determine another kind of fate.
The seal was also used in the early church to show the power and authority of a bishop or the pope, and it carried with it the same significance as if the official was speaking the documented words in person, and the one bearing a sealed document was entitled to all that was given under that seal. Even today seals are used to autheticate documents, or to certify the identity of the licensed professional giving certain assurances.
Paul uses such a seal to illustrate what happens to you when you become a believer in Jesus Christ. From the moment of your acceptance of His salvation, and your commitement to surrender your life to Him, the Holy Spirit "took up residence within you", as a sign of the power and authority of God. You became His possession, sealed forever in His kingdom. He has made you His and you have become a part of His kingdom. Since you have the seal of Christ, you are entitled to all the benefits given under that seal.
I am of the belief that once you are sealed by the Spirit, that seal can never be broken, even if you fall away by being disobedient to God. It is that very seal that will always pull you back into right living and right standing with Him. Once you know the truth of Jesus Christ, you will always believe, and those who claim that they once were followers of Christ, but now are not, probably never truly believed in the first place. Jesus radically changes you, and once He has, you will know it.