A “saved” person can obviously fall away beyond redemption, else the Scriptures wouldn’t have mentioned it. The question to ask is, How can one lose his salvation?
By sinning? Certainly not! — 1 John 1:9, as you mentioned.
So if not by sinning, then how?
Well, Jesus did say there was one unpardonable sin — blasphemy against the Spirit (see Matthew 12:31-32 and Luke 12:10).
What is blasphemy? Blasphemy means defiant irreverence or cursing God or degrading Him. Even relating evil to God.
How does a “saved” person do this? Well, it’s pretty hard. You have to make a deliberate decision that, despite all you know about God, you want nothing more to do with Him. You need to prayerfully, or more likely with clenched fists raised against Him, tell God to get out of your life once and for all. Say, *I REJECT YOU!*
Hard to imagine even the most backslidden person doing that. But if anyone can ever get to that point, then the impossibility of their redemption (Heb 6:4-6) isn’t hard to fathom at all.
Bottom line: You can’t lose your salvation by sinning. Not even by sinning a whole lot. To lose your salvation, you have to really *want* to, making a deliberate decision to reject God and tell Him to get out of your life for good.
The tl;dr version is: Anyone who is worried about it, hasn’t lost his salvation, else he wouldn’t be worried about it.
Hope this helps.
Heb 6:4-6 is one of the few passages where the meanings are much better understood after a verse-by-verse, word study from the Greek.
Similar to parables, those who fall out of fellowship thinking the passage indicates we can lose our salvation, will not understand the passage anyway. Those who remain in faith, will grow even more from the passage.
The passage provides sound doctrine upon which to understand efficacious grace.