Q: I do not believe that “once saved always saved” is even in the Bible. If a saved person commits serious sin and dies in that back-slidden state — without repentance — will they go to Heaven or Hell?
This may be an appropriate place to discuss the sin of “blasphemy of the Holy Spirit” and what exactly Jesus meant by that.
“Truly I say to you, all sins shall be forgiven the sons of men, and whatever blasphemies they utter; but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin” (Mark 3:28-29)
It seems likely that what Jesus was talking about includes losing your salvation in this passage. He said that all sins will be forgiven except the blasphemy of the Holy Spirit.
What could He possibly mean by this? If a person was saved, and then committed the blasphemy of the Holy Spirit, then they would not be forgiven. This seems to be consistent with the passage.
You mention “serious sin.” I think the question is, “What is the blasphemy of the Holy Spirit?” Here is my take on this. The “blasphemy of the Holy Spirit” is to deny God in such a manner so as to completely shut out any conviction of the Holy Spirit. Some of the Pharisees were no doubt guilty of that (Jesus said to some of them, “You are of your father the devil,” in John 8:44).
So is “once saved always saved” in the Bible? On the one hand, according to John 10:27-29, it is.
“My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, 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)
But it appears to be conditional upon some form of conviction of sin remaining in that person, and if the person has some form of conviction of sin, it seems evident that they have not committed the sin of the blasphemy of the Holy Spirit (so if you are afraid you have committed it, you have not committed it).
Some of the Corinthian believers were engaging in unholy practices during the Lord’s Supper. They were judged as a result and some died. There is no indication that these lost their salvation. However, they died prematurely and (it seems apparent) still went to heaven.
“For he who eats and drinks, eats and drinks judgment to himself if he does not judge the body rightly. For this reason many among you are weak and sick, and a number sleep.” (1 Corinthians 11:29-30)
In summary, then, if you have any conviction of sin, you have not committed the blasphemy of the Holy Spirit. What you need to do is to confess your sins to God, who then immediately forgives you.
“If we confess our sins, He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness.” (1 John 1:9)
While that my seem “unfair” of Him to forgive you immediately like that, this is what the Scripture teaches. You would be in sin to doubt that He truly forgave you after you had confessed your sins to Him. Therefore, your next step should be to praise Him and thank Him for His faithfulness and goodness to you, and especially for His forgiveness to you.