Some fellow believers in Eternal Security won't fall into the trap of questioning someone's Salvation based on behavior. But they may insist that a Truly Saved person won't hold views that disagree with The Bible.
They base it mainly on verses like "Everyone who is of the Truth hears my voice" in John 18.or John 8:47 "He that is of God heareth God's words: ye therefore hear them not, because ye
are not of God.".
The danger of this view is that it's very strongly based on opinion. Yes I believe every Biblical question has only one correct answer. But the Brain is still of the Flesh. The Faith that saves is of the Heart, not an intellectual belief. The thief on the Cross didn't even expect his Belief would save him.
Basically this view says a Saved person can be mistaken on something at first, but if you show them from The Word the truth and they don't believe it they aren't Saved.
They may add that a Saved person can be stubborn on maybe 1 or 2 issues, but if they do it repeatedly they're not Saved.
The main Pastor I saw teaching this, tied into it his view that the Disciples (most anyway, not Judas who was never Saved) were already Saved all through Jesus' ministry. Saying they were Saved before Jesus called them. Well the Disciples repeatedly refused to accept what Jesus told them, that he had to be killed and rise again.
In my view, if someone outright doesn't care what The Bible says then maybe it can be logical to question their Salvation. But if they clearly care what it says, if they're doing mental gymnastic to get around a clear teaching. Or just insisting that you must be misusing that verse without an argument at all, then it sounds to me like they are hearing The Word, just struggling with accepting it like the Disciples did.
I think It's pretty difficult for an unsaved person to even believe The Bible is infallible. One might say they view it as God's Word but have some qualifiers to that which make it fall short of true infallibility.
The World makes it pretty hard to simply see The Bible as Infallible. Historically when someone has come to the conclusion that The Bible is a perfect book for purely logical reasons, they get Saved almost immediately.
Reason why I'm so annoyed about all these tests Christians throw out to prove someone isn't Saved, is that the Bible is far less ambiguous on what Proves someone IS Saved.
1 John 4:2 says " Hereby know ye the Spirit of God: Every spirit that
confesseth that Jesus Christ
is come in the flesh is of God". And verse 15 says "Whosoever shall
confess that Jesus is the Son of God, God dwelleth in him, and
he in God.".
Romans 10 says "All who call upon the name of The Lord shall be Saved" no qualifying statements.
Those who take the verses on "knowing a tree by it's fruits" to say an Unfruitful Christians can't be saved are misunderstanding the verse. BUT if a believer has borne visible Good Fruit, then that person's Salvation certainly should not be questioned.
I would also never question the Salvation of anyone who died for their Faith in Jesus. Yet many Catholics have done that, especially during the French Revolution. And none are more doctrinally wrong then Catholics.