Can a true believer turn away from God at some point and eventually commit blasphemy against the Holy Spirit? I don't believe a true Christian would be capable of that no matter how far they strayed because one saved, always saved, but I need verses to support my opinion to share with someone else.
Thank you for your question. The "unpardonable sin" of blasphemy against the Holy Spirit is mentioned in the three synoptic Gospels: Matthew 12:31-32, Mark 3:28-29, and Luke 12:10. Historically, these verses have aroused a great deal of anxiety and fear, especially in those with a sensitive conscience. But what do these sayings mean?
In my opinion, the two best positions are the following:
If the first option is correct, some would hold that it is not even possible to commit this sin today. In this view, this sin could only have been committed while Christ was physically present on earth and performing miraculous feats through the power of the Holy Spirit. Others would hold that the sin can be committed today; nevertheless, there is a pretty large consensus among evangelical Christians that a true believer could never commit this sin. After all, Peter says that all true believers "are protected by the power of God through faith for a salvation ready to be revealed in the last time" (1 Pet. 1:5). And Paul tells the Philippian believers that he is "confident of this very thing, that He who began a good work in you will perfect it until the day of Christ Jesus" (Phil. 1:6).
Although I may certainly be wrong, I honestly prefer the second view. Please notice that if this view is correct, a true believer could not possibly commit this sin by definition. While I could list many reasons why I prefer this view, let me mention just a few.
First, it is by far the easiest way to make Scriptural revelation self-consistent. For instance, we know that persistent unbelief is an unpardonable sin. But Jesus says that all sins and blasphemies will be forgiven except blasphemy against the Holy Spirit (Mark 3:28-29). Logic, then, seems to require that blasphemy against the Holy Spirit is persistent unbelief.
Second, notice the progression of ideas in Matthew 12:30-33. Jesus begins by stating the importance of being rightly related to Him (v. 30). He then describes the unpardonable sin (vv. 31-32). He then seems to present His listeners with a choice: "Either make the tree good...or make the tree bad; for the tree is known by its fruit" (v. 33). Could Jesus be offering those who had spoken against Him in v. 24 (they are the ones He is speaking to - v. 25), an opportunity to repent (i.e. change their minds about His identity) and become rightly related to Him in v. 33? If so, it would seem to indicate that blasphemy against the Holy Spirit is persistent unbelief. And the cure is faith, leading to forgiveness.
Third, although Mark's parenthetical explanation in 3:30 could be taken as evidence of the first view; nevertheless, I see in it evidence for the second view as well. After all, if they were saying that Jesus "has an unclean spirit" (v. 30), it certainly indicates that they did not believe Him to be who He actually was (and is). Thus, this statement is consistent with simple unbelief in the person of Christ.
Finally, why doesn't John mention this sin? It certainly seems like it would have been important. But what if he did mention it, but simply described it differently? Look at John 16:8-9. Jesus is speaking of sending the Holy Spirit after His ascension. Notice what He says of the Holy Spirit: "And He, when He comes, will convict the world concerning sin, and righteousness, and judgment; concerning sin, because they do not believe in Me...." The Holy Spirit convicts the world concerning the sin of unbelief, or lack of personal faith, in Jesus! Could the persistent rejection of the Holy Spirit's conviction, and the willful refusal to believe in Jesus, thus be blasphemy against the Holy Spirit? That, at any rate, is my opinion. Thus, by definition, it is absolutely impossible for a true believer to commit this terrible sin. It can only be committed by someone who persistently rejects the convicting ministry of the Holy Spirit, choosing to remain in their unbelief.
Additionally, this ties in very well with what is said in other parts of the New Testament concerning the ministry of the Holy Spirit in the life of the believer. All true believers receive the Holy Spirit (Rom.8:9, 14). The Holy Spirit testifies that believers are God's adopted children (Rom. 8:16). The indwelling ministry of the Holy Spirit in the believer's life is said to be permanent (John 14:16-17), a pledge or "down-payment" of an eternal inheritance (Eph. 1:13-14). Indeed, the Holy Spirit is said to "seal" believers "for the day of redemption" (Eph. 4:30)!
Just a very few of the many good passages on the security of the believer can be found in Rom. 8:28-39; John 10:27-30; and 1 John 5:9-13. But my own favorite is John 6:35-40. Read this passage carefully. Notice v. 37, that the one who comes to Jesus will certainly not be cast out. Notice that Jesus came to do the will of His Father (v. 38). But what was His Father's will? That the Son lose none of those who come to Him (v. 39)! But think about this. If Jesus loses even a single one who truly comes to Him for salvation, then He has not fulfilled the Father's will! But this is impossible for Jesus always does what is pleasing to His Father (John 8:29). Thus, it is impossible that Jesus will lose any who come to Him for salvation. Thus, Christians cannot commit the unpardonable sin.
Hope this helps. God bless you!