Maybe we sometimes try to overthink this whole situation.
Hebrews 10:14 says that by one sacrifice Christ has made us “perfect forever.”
How complete is “perfect?” How limitless is “forever?” Both terms are superlatives; there is nothing additional that can be added.
If we have been made perfect forever, then we are forever perfect. Right? It seems simple and direct. And if we have been made perfect forever, then we can never become imperfect at any time. Also, true?
So if we are perfect forever, then it must be impossible that we could fall into and out of righteousness. There just doesn’t seem to be much room for misunderstanding or debate.