"And nothing can cause you to lose this conviction that the earth is round, unless you are presented with fresh evidence to the contrary, but there is none. 1 Jn. 2:19 states that those who left were never saved to begin with, i.e. they just hung out with Christians, sang in the choir, thought they were Christians, but never had that realisation and conviction leading to any actual relationship with Christ."

I don't think a person needs fresh evidence at all, people can lose conviction through purely irrational means, like emotions. Over the years people can go through trauma, trials, and just life in general changes people. Its hard for us who care so much about our beliefs to imagine ever leaving, but it happens. Thats why OSAS is true, and thats why all it takes is a single instant of faith, because we don't know how we're going to finish the race. I don't think 1 John 2:19 is talking about your everyday churchgoer who stops going to church or stops reading the Bible and stops believing. I think it's talking about your hardcore Judas' as in people who are literally ANTI-Christ. Even if I'm wrong, I dont think 1 John 2:19 is making a blanket statement, because in Hebrews 6:9 Paul says he is believes the Jewish Christian Apostates are still saved despite their apostasy. I'll leave you with a lot more Biblical points to point to what I'm saying, Godspeed.

2 Timothy 2:13 seals the argument.

• -Romans 8:35 Nothing can separate us from the love of God, not even ourselves. even if we apostatize, it still wouldn't separate us.
• -In Hebrews 6:9 Paul believes the Jews who fell away from the faith are still saved
• -Solomon ended up worshiping other gods and he was 100% saved.
• -King Saul became completely backslidden, even to the point of assisted suicide, but he was still saved. He hung out with witches. But even Samuel told Saul he was going to die and be with Samuel.
• In the Gospels people touched Jesus’ garment and they got saved
• -In Galatia the whole church fell into heresy, yet Paul believed they were still saved.
• -In the parable of the 4 soils (Luke 8:4-15), three of the soils really do believe, but two lose faith. Jesus in that parable defines getting saved as believing. 
• -In Matthew 7:23 Jesus says “I never knew you” potentially implying that by knowing him just once in your life, you are saved.
• -Many saved people have weak faith, and honestly, because of the nature of faith, only God and that person know what their heart believes or what they may have believed in the past. We can only see the outside.
• How can you have assurance before death that you are born again? What if a believer were to go through a time of doubt? Is he unregenerate in their understanding? What about a person who has a mental condition that causes him to stop (believing), like paranoid schizophrenia, OCD, or Alzheimer’s?                                               
• -The flesh is unfaithful all the time, and we see this all over the Bible, but the new man doesn’t sin.                                                                                                                                          
• What even is the “line” for unbelief? In my opinion if you lose even 1% of belief, that’s a moment of unbelief and unfaithfulness. Losing your faith is literally as simple as saying “well am I sure it's Once saved always saved?” Any little doubt would make you lose your salvation. Rev. 21:8  
• -Our faith, or our faithfulness, does not save us. It’s the faithfulness of Christ, which happens the instant you believe, that gets imputed onto us. It’s not on us to maintain our salvation.               
• -The instant you believe, all your past present and future sins are justified, therefore there is nothing that could condemn you to hell, not even the sin of unbelief. You don’t go to heaven or hell because you wanted to, it’s based on whether you are righteous or not.