I have heard of a few people in my life who appeared to be devout Christians, who later rejected Christ and become devoted atheists and enemies of the Faith. According to 1 John 2:19, it would appear these people never were saved to begin with. Is that the correct interpretation? In light of this, how can anyone be certain of their own salvation?
There are two possibilities here. The first is John’s statement to the effect that leaving the church can be a confirmation that a person never truly belonged to it (1 John 2:19). The other possibility is that they’re like the sheep who wander off in the Parable of the Lost Sheep (Luke 15:3-7). The Lord knows our heart and regardless of what impression our words and actions convey, He has promised not to lose even one of those He’s been given (John 6:39-40). At the appropriate time He will go after those who are His and bring them back.
I’ve met people who have really huge issues with the Lord for one reason or another and can’t find anything good to say about Him. To all the world they look and act like militant non-believers. But people who really don’t believe in God are more likely to be indifferent toward Him because you can’t have issues with someone who doesn’t exist. This has led me to conclude that a person is either forever saved or was never saved, and lot’s of times it’s impossible to tell which it is.
As for being certain of our own salvation, I think it’s safe to say that wondering if you’re really saved is a good sign that you are. Otherwise you wouldn’t care.