It is saying, we are sealed/marked by the word of God we receive in our heart, and that word, which is a seed/offspring of and from the holy Spirit. And that said word or seed of the spirit of God is a down payment or pledge until the day of our redemption from The works of the flesh.
The promise and seal is good, so long as you keep the word of God in your heart, until you die or are taken up in a rapture. The word of God is both the flesh and blood of Jesus, and to reject the gospel, or the word of God and its promises, is to reject that which you were sealed or marked with, that being, the seed or offspring of the holy Spirit.
This only applies to mature Christians, like the former ordained Baptist minister I spoke with who turned atheist after he allegedly died three times. One of which was for an hour and a half, and all he saw was darkness, and because of that, he rejected the word of God and Christ, along with his wife, who also was an ordained Baptist minister. They were sealed with the promised spirit, but because they rejected both the gospel and Christ, which one must have in order to have the flesh and blood of Jesus, which is life, lost their salvation. For the promise is indeed forever to those who keep the word to the end.
