Fallacy. Those who teach it teach heresy. One must work on one’s salvation every day and pray for perseverance to the end. I recommend the following answer: https://www.catholic.com/magazine/print-edition/once-saved-always-saved
That was an adequate explanation, but I prefer reading about Tolkien’s Eucatastrophe. The missive quotes Tolkien and discusses the role of Fantasy literature for him and for all Catholics who are, or might be, interested in fantasy.
My little Alexander is crazy for this kind of literature, especially if it’s bloodthirsty. Perhaps, that will bring him to faith. I don’t know, I’m just an alien observer.