The Bible does teach that God's people are sealed, and the Holy Spirit is God's gift and mark upon those who belong to Him. But we must be careful not to add to what Scripture actually says.
Jesus never taught that once a person believes, nothing he does afterward can ever affect his relationship with God. Instead, Jesus repeatedly told His followers to remain in Him, keep His words, bear fruit, overcome, and endure to the end.
Jesus said, “Abide in Me, and I in you. As the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in Me” (John 15:4, NKJV).
Then He gave an even stronger warning: “If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned” (John 15:6, NKJV).
So the question is not whether Satan can break God's seal. Of course he cannot. The question is whether a person who has received God's Spirit must continue to abide in Christ. Jesus says that we must.
Jesus also said, “But he who endures to the end shall be saved” (Matthew 24:13, NKJV).
And look at the Book of Life. Jesus said, “He who overcomes shall be clothed in white garments, and I will not blot out his name from the Book of Life” (Revelation 3:5, NKJV).
Notice the words: “He who overcomes.” The promise is connected with overcoming and remaining faithful.
Jesus also said, “Be faithful until death, and I will give you the crown of life” (Revelation 2:10, NKJV).
Therefore, God's seal should never be turned into a teaching that says our future choices no longer matter. God gives us His Spirit, but Jesus tells us to remain in Him. God gives the promise, but Jesus tells us to endure. God offers eternal life, but Jesus warns us not to turn away.
The Bible never teaches that we can break God's seal by our own power. The final judgment belongs to God. But neither does Jesus teach that a person can receive the Spirit and then abandon Him without consequence.
The message is much simpler: Believe in Christ, receive His Spirit, remain in Him, keep His words, bear fruit, overcome, and endure to the end.
The security is not found in saying, “I believed once, therefore nothing I do afterward matters.” Our security is found in continuing to walk with Christ and remaining faithful to Him until the end.
i will post about seals in more details in a new thread