Some who profess to be Christians may, after many years as a practising Christian, later renounce their faith and change to another religion, or call themselves atheists. In fact, we might say that many people do this. This seems to pose a problem for Christianity. Since I have said “If you seek God, He will reveal Himself to you,” one who has been a practising Christian and now isn't might say, “Well, I did seek God and ultimately I found nothing.”
Let's first establish the Biblical doctrine of “eternal salvation”. That is to say that once a person is saved, it is impossible for them to become “unsaved”, or to lose their salvation. Before we come to Christ we are presently “dead in trespasses and sins” (Ephesians 2:1). But when we come to Christ we receive, at that moment, eternal life...
My sheep hear my voice, and I know them, and they follow me. I give them eternal life, and they will never perish, and no one will snatch them out of my hand. My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand. (John 10:27-29)
This passage tells us that followers of Christ will “never perish.” Those who do not follow Christ will do, in that “eternal death of the soul” sense of the word. But here we read further, regarding those who are followers of Christ, that “no one will snatch them out of [Christ's] hand.” This is certain because, as the passage says, the Father “is greater than all”, and won't allow it.
Let's now back up a little and see this passage in its broader context...
So the Jews gathered around him and said to him, “How long will you keep us in suspense? If you are the Christ, tell us plainly.” Jesus answered them, “I told you, and you do not believe. The works that I do in my Father's name bear witness about me, but you do not believe because you are not part of my flock.” (John 10:24-26)
The Jews to which this passage refers are certainly those who thought that they were right with God; they attended the synagogue, read the Scriptures, and worshipped God. By all external appearances they were God's people. But nevertheless, they were “not part of [Christ's] flock.” The same thing can happen in the Christian community. Truly being one of Christ's followers is a matter of the heart; having a true faith in the reality of Christ's Lordship. If you truly believe He is Lord, then you will follow Him as your Lord. To me, ceasing to believe in Christ is like ceasing to believe in people. The only people who could do so are those who have never really experienced a relationship with Him. If they had, I don't see how they could ever deny the reality of Christ. But there are many who attend Church but do not know Christ. They may have been brought up in a Christian home like I was, and attend Church simply because they've always been taken there by their parents. Or they may have gone with a friend as a social activity, and become involved in the community as a social community. There are those who will even say explicitly things like, “It is good to be involved in Church because of the morals that are taught there... but that Jesus Christ was anything more than a man is not true.”
Such a person, who claims to be a Christian but is not truly following Christ in their heart, is not in fact seeking God. This is certain because they either think that there isn't really a Christ to be found, or they think they have already found Him; and they are not seeking Him daily. If they were, that would be following Christ. True Christianity is about seeking God daily. We seek God's will in all things.
Now I'm not saying that there aren't some hard cases which can trouble us; people who deny Christ, yet we can hardly believe that they weren't true Christians at one time. Those Jews looked like genuine believers, but upon closer examination they weren't because they did not believe Christ.
Now think about this. How does a person go from being a new-born Christian to a person of mature faith? All along the way their faith will have been challenged. God does this so that we are able to grow in faith. He is our teacher. If I can only speak from my own experience, then my own experience is this; that I am constantly being challenged by the Word of God, but I persevere in trying to understand, knowing that God is teaching me about Himself. This is not a case of “This doesn't make sense, but I will stubbornly keep on believing until I find some way of explaining away this perplexity...” In fact, I'm not content with any old explanation for some of the difficulties I have with Scripture. I want nothing short of the truth. But I persevere because I know it is true, and that my understanding is not perfect. And I persevere because of the countless number of times my confusion about the Word of God has been resolved. When I have persevered in seeking an answer I have always found one within the Bible itself, so that I am confident that whatever questions I still have, God will give me an answer. But we cannot know everything all at once. The one who rejects Christianity for lack of answers is not seeking God, and demonstrates that they have not known Him.
It is written in the Prophets, “And they will all be taught by God”. Everyone who has heard and learned from the Father comes to me (John 6:45)