"In Him (Jesus Christ) you also trusted, after you heard the word of truth, the gospel of your 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 the purchased possession to the praise of His glory." (Ephesians 1:13-14 - Also Read About "God's Guarantee" In 2 Corinthians 1:22 and 2 Corinthians 5:5)

Notice that the sealing of the Holy Spirit is also called 'God's guarantee', which ensures that salvation (or 'our inheritance') will remain secure (or 'sealed' inside the believer) until Jesus returns to receive His purchased possession ... which is the Church, consisting of all those who have placed their faith in Christ. Contrary to popular yet false church teaching, this 'seal' cannot be broken by a lack of righteous works, poor behavior, failed Christian service ... or by sin.

"For sin shall not have dominion over you: for ye are not under the law, but under grace." (Romans 6:14)

"Stand fast therefore in the liberty by which Christ has made us free, and do not be entangled again with a yoke of bondage." (Galatians 5:1)

After Pentecost - Jesus now lives inside the Christian believer as "The Spirit of Christ" (another name for the Holy Spirit) and promises that He will never leave or forsake them ... meaning that He remains forever. This promise, spoken before His crucifixion, reaffirms that the "sealing of the Holy Spirit" is truly "God's guarantee" ... where salvation, once received, is now 'sealed with the Holy Spirit living inside the believer' and cannot be lost. Under certain conditions our sin can "grieve the Holy Spirit", which is something every Christian should avoid. Yet, unlike the way He operated under the Old Testament program ... this grieving will never 'break the seal', causing the Holy Spirit to leave. Saint Paul said it this way...

And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption. (Ephesians 4:30)

Many churches and mainline denominations (but thankfully not all) quietly reject Christ's promise of a secure salvation because it exposes the falsehood of their out-of-context theology and Bible teachings. This is why so many attempt to discredit or redefine this incredible truth of the authentic Gospel - which is actually a cloaked denial of the finished work of Jesus Christ.

"And I will pray the Father, and He will give you another Helper, THAT HE MAY ABIDE WITH YOU FOREVER — the Spirit of truth, whom the world cannot receive, because it neither sees Him nor knows Him; but you know Him, for He dwells with you AND WILL BE IN YOU." (Words of Jesus in John 14:16-17)

After His resurrection, Jesus said...

"...and lo, I am with you always, even to the end of the age." Amen. (Matthew 28:20)

Later, Paul quotes Jesus as saying ...

"Let your conversation be without covetousness; and be content with such things as ye have: for he (Jesus) hath said, I WILL NEVER LEAVE THEE, NOR FORSAKE THEE. So that we may boldly say, The Lord is my helper, and I will not fear what man shall do unto me." (Hebrews 13:5-6)

"For by grace you have been saved through faith, and that not of yourselves; IT IS THE GIFT OF GOD, not of works, lest anyone should boast." (Ephesians 2:8-9)

"For God's gifts and His call are irrevocable. [He never withdraws them when once they are given, and He does not change His mind about those to whom He gives His grace or to whom He sends His call.]" (Romans 11:29 Amplified Bible)

What? know ye not that your body is the temple of the Holy Spirit which is in you, which ye have of God, and ye are not your own? For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God's. (1 Corinthians 6:19-20)

"But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ from the dead will also give life to your mortal bodies through His Spirit who dwells in you." (Words of Saint Paul from Romans 8:11)