I do not know that "they" exist except in theory.
Can you see the soul of another and know if that person is saved?
The Apostle John says: They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us. (1 John 2:19) so I believe THAT is a fact. Jesus warned me about false Prophets (“Watch out for false prophets. They come to you in sheep’s clothing, but inwardly they are ferocious wolves. By their fruit you will recognize them. Do people pick grapes from thornbushes, or figs from thistles? Likewise, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus, by their fruit you will recognize them.) in Matthew 7:15-20 and Jesus warned me about false Disciples (“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven. Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?’ Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’) in Matthew 7:21-23, so I believe THOSE are a fact.
These people that are not saved - then saved - then not saved again ... are only hypothetical to me. I only know of my salvation and how God has jealously protected it and prevented me from stumbling beyond his grasp. The shepherd left the 99 and dragged the one back.
So I am forced to accept what I read in the Bible and know for myself.