Scripture is clear. When a soul believes in Jesus, he or she has (present tense) eternal life. From the moment of faith, we are eternally secure.
Jesus said to them, “I am the bread of life; he who comes to Me will not hunger, and he who believes in Me will never thirst. But I said to you that you have seen Me, and yet do not believe. All that the Father gives Me will come to Me, and the one who comes to Me I will certainly not cast out. For I have come down from heaven, not to do My own will, but the will of Him who sent Me. This is the will of Him who sent Me, that of all that He has given Me I lose nothing, but raise it up on the last day. For this is the will of My Father, that everyone who beholds the Son and believes in Him will have eternal life, and I Myself will raise him up on the last day.” John 6:35-40; Emphasis Added
“for the gifts and the calling of God are irrevocable.” Romans 11:29; As is His love, cf. Romans 8:35-39
“In Him, you also, after listening to the message of truth, the gospel of your salvation, having also believed, you were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory.” Ephesians 1:13-14 cf. 4:30
All that said, there are clear warning passages (e.g., John 15; Hebrews 6 & 10; 1 Corinthians 9:24-27), and two very scary statements:
“Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter. Many will say to Me on that day, ‘Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?’ And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness.’ Matthew 7:21-23
“Children, it is the last hour; and just as you heard that antichrist is coming, even now many antichrists have appeared; from this we know that it is the last hour. They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us.” 1 John 2:18-19
I would like to set aside the warnings that were clearly written to believers, dealing with temporal discipline or loss of kingdom rewards. To address the issues of sincere, but false, belief and apostasy.
I do not see that the Bible teaches that a believer can “sin” his way to eternal damnation. Both James and 1 John end with admonitions to go after sinners to save them from physical death. Paul twice condemns specific sinners to the destruction of their flesh for the salvation of their souls. (The man living with his mother-in-law, 1 Cor 5:5; and the two teachers who veered from the truth, 1 Timothy 1:18-20.)
All of that said, Jesus warned that there would be people who believed themselves to be Christians and yet were turned away. And there are others who proved they were never Christians by leaving. While this may be an oversimplification, let’s call the first group “false believers” and the second group “apostates.”
Jesus said of the false believers that He never knew them; that the ones that can enter the Kingdom, the ones He knows, “do the will of His Father.” So, what is the Father’s will? In John the Father’s will is belief in the Son! Every other precept concerning the “will of God” is directed toward those already in God’s family, so it follows that the “will of God” Jesus addresses in Matthew 7, is faith in Son – the Gospel. So, who might believe they were serving Jesus, without believing in Jesus? 1 John supplies us with an answer. It has to do with the version of Jesus in which you believe – do you believe in Jesus that is fully God and fully man? (4:1-6; 5:1-3). Millions of Mormons and JW’s are sincere in their faith, but wrong about Who Jesus is. Add to that many others who are in false churches, or who are in Gospel preaching churches, but are “faking” it to fit in. It’s not hard to see who those disappointed “false believers” might be. So, what about the apostates?
At some point in the history of the Christian cults and uber-liberal churches, someone who was an apparent believer led folks astray. John says of them, they proved they weren’t part of “us” because they never came back. For those of us who love someone who has “walked away”, herein lies the hardest truth of all. There have been (and very possibly are) those among us who never were justified/regenerate; not because God failed, but because they refused to believe (John 3:36 – the word translated “does not obey” in the NASB is the from the Greek word for unbelief (apeitheó). The best modern English translation of that word’s contextual meaning is “reject.” So, in John 3 the teaching is simply God offers you salvation through faith in Jesus, and if you do not believe you are already condemned because you have rejected His offer. When combined with John’s epistolary statement about false teachers going out and proving they were never a part, we can see that, despite testimony and apparent fruit, there will always be tares (faith rejecters) among us.
Conclusion: Believers (wheat) and unbelievers (tares) can look an awful lot alike. As we look out on our congregation, class, or family (wheat crop), we need to be mindful that some of them might be tares.* While people are still in hearing range, we need to be continually lifting the cross and praying for salvation and spiritual growth here, in the visible family. After someone removes themselves, we keep right on praying! Ultimately, we are all in God’s hands, so what better place to leave our “denying” loved one than in the hands of our Creator/Redeemer? It’s not His will that any should perish (1 Timothy 2:4; 2 Peter 3:9).
*Or wolves (Acts 20)