Sounds like a work to me. You have to hold it fast. If you don't, you lose it.
Exactly the point of Scripture.
As the context of that parable demonsrates, those who have good soil, meaning good hearts, will hold fast the word of God; so they will continue to believe the word of God. That verse that doesn’t say that doing good works earns someone salvation.
If you consider a continual belief in God as a work then fine - that is semantics. The point is a true belief in God and not a superficial belief in God is what saves a person. You can tell those who have a true belief in God because they will bear fruit. All others do not have a solid belief.
That’s it - it’s that simple. True belief leads to good works. A person doesn’t need to do good works to earn their way into Heaven. They should do good works because they have a relationship with Christ who gives power to his followers to do good.