I put forth a poorly formatted (lost the original formatting when pasting and didn't notice before editing ability lapsed), [too] lengthy thoughts post earlier. Rather than starting from the loss of salvation OP like so many others that end up in the typical mass of repeated disagreement, although not unique, I turned it back to a question of who truly believes and is true belief determined by God rather than just viewed loosely as it seems is normally the case.
I'm not aware of any one verse that specifically breaks down a Deific definition for belief/faith that would be a departure from what was normally understood by the ancients. To them, it was works-based from the perspective of Israel and from paganism. However, the body of Christ is not comprised only of Israel, but mostly Gentiles who did not come from the works-based system of the Mosaic Law, which carried over into the Kingdom Gospel, but from their pagan grounding that had variations of works not holistically defined for all as was the case with the Mosaic Law.
In answer to your question here re: the Sealing, in light of what I just said, I'd answer in the same track, who truly gets sealed?
In further answer and more globally, I think the whole works salvation argument is overdone and many remain on the outward ranges of the pendulum swings both ways. Most errors are met with counterarguments that also end up being erroneous.
Then, yes, salvation in its full scope involves good works we were created to do under grace in Christ in Spirit meaning apart from God we can do nothing but in Christ we can do all things including work with God to accomplish our salvation that is first and foremost His salvation that He gifts to men.
The only caution I would cast into that is the Calvinistic bent that many now throw into the pot that is a strong tendency for so many...so much so that it has regained popularity moreso than rightly dividing the word of truth. The blend of their theology is, as before, a mixed mish-mash of gospels and theological mind and word games to try and make it palatable to more people. It tends toward removing responsibility from the individual believer for their faith, claiming that it's ALL the Lord's doing, which is the reason they are told by others that their theology begs the question: Why should any of us engage sharing the confused mixture of their gospel if none can even possess faith in the first place.
I'm not sure if you've kept your fingers in the pulse of popular doctrines and dogmas out there, but much of evangelicalism has gravitated over to the side of Calvinism, which has tainted a number of unbelievers I have encountered who want nothing to do with the Bible because of the hypocrisies inherent within the various inclusions of Calvinistic warps and twists. That's not to say that everything he taught was wrong, but you know what just a little leaven does to the whole loaf...
MM