Now that two public Christians have renounced their faith, some thoughts about the other 99% of us:
Apostasy is a possibility for every single Christian.
Every single Christian is at risk.
Every single Christian is in constant risk.
Now: How can I, who believe in eternal security, say this? Aren’t we supposed to tell everyone that “I have absolute security of my salvation?”
The doctrine of “once saved, always saved” in no way eliminates the doctrine “there are people who have profess Christ, are baptized, join the church, do good works, and then walk away from it.” Apostasy happens all the time, but the thousands of other people who slipped away this month may have just kept it to themselves.
What confuses us, at least in the tradition I grew up in, is that we have a messed-up set of labels. I was taught that, if a person wanders away from church, from walking with Christ, from faith, that that person is “backslidden”. But I cannot find in the Bible that “backsliding” in that sense is in the vocabulary of the gospel. Sliding away from Christ is not a case of someone saying something like, “Well, I was on a low-carb diet, but I backslid!” No, when a person rejects Christ – or passively, just slips away from Christ – that person is in eternal peril. I refuse to give any comfort to a backslidden person, only warnings of danger! In fact, I know of no way to distinguish between backsliding and apostasy, except that the apostate is possibly more open about his or her state. In that case of Joshua Harris and Marty Sampson, I am horrified that they lost their way – or tossed it away – but at least they were frank about it.
I read one article by a man who suggested that Harris couldn’t have really been serious, that he would snap out of it. What an odd interpretation of someone who has openly renounced Christ! The Bible has it that Christ will renounce him before God on Judgment Day, period (Matt 10:33).
Another problem, and much more toxic, is this idea that, once you raise your hand to accept Christ, it does not matter what happens from then on, you are saved eternally.
Although I would rather not mention his name, I am obliged to quote Charles Stanley, who teaches a serious error:
God’s love for His people is of such magnitude that even those who walk away from the faith have not the slightest chance of slipping from His hand.… [We] are not saved because we have an enduring faith. We are saved because at a moment in time we expressed faith in our enduring Lord…Even if a believer for all practical purposes becomes an unbeliever, his salvation is not in jeopardy…believers who lose or abandon their faith will retain their salvation, for God remains faithful. 
Please read that again carefully.
I looked around to see if Charles Stanley has recently commented on Joshua Harris’s or Marty Sampson’s defections, but found nothing. However, I can see no way out of saying that according to his doctrine it should be framed like this:
Even though Joshua Harris is now an unbeliever, his salvation is not in jeopardy. Marty Sampson was saved at the moment that he expressed faith in our enduring Lord; he has lost his faith but will retain his salvation.
And to be logically consistent, we should rewrite the New Testament texts as well!
Those that endure to the end will be saved. But, no problem! So will those who do not endure to the end.
Timothy, some have rejected the truth and suffered shipwreck in regard to their faith, but no problem! We can raise the Titanic!
…you tolerate that woman Jezebel, who calls herself a prophet and is teaching and beguiling my servants to practice fornication and to eat food sacrificed to idols. I gave her time to repent, but she refuses to repent of her fornication. But, no problem! She said she believed in Christ, so no Lake of Fire for Jezebel!
(Matt 24:13, 1 Timothy 1:19, Rev 2:20-21, very broadly paraphrased!).
The Reformed doctrine, correctly I believe, teaches that “endurance” in the faith is a necessary part of security of salvation. My security does not lie in my feelings. My security is in Christ. If I am not in Christ, I have zero security. (This works equally well for the genuine believer who is having doubts, although this is a topic for another day!) And when we share our faith and ask the question, “Can you know that are saved?” our position should be, “Yes, since I know who Christ is”, not “Yes, because I know my own heart!”
Charles Spurgeon expresses this wonderfully, and he was a man who most certainly believed in eternal security: “true believers shall not apostatize, but shall stand fast, and shall be kept even to the end.” Still, he warned his flock against apostasy! “…it is a great and solemn truth that every child of God will hold on until the end, but it is an equally solemn truth that many who profess to be the Lord’s are self-deceivers, and will turn out apostates after all.” 
His thoughts in Morning and Evening for the morning of January 19 are sublime, here is the link.
Cling to Christ!
 Charles Stanley, from the chapter “For Those Who Stop Believing,” in Eternal Security: Can You Be Sure? (Nashville: Nelson, 1991), 74, 80; also from the chapter “Faithful to the Faithless,” 93, 94.
 Spurgeon Quotes on Apostasy, https://www.girdedwithtruth.org/spurgeon-quotes-on-apostasy/
“Apostasy, Right Ahead!” by Gary S. Shogren, Professor of New Testament, San José, Costa Rica