Mark 3:28 I tell you the truth, people will be forgiven for all sins, even all the blasphemies they utter. 29 But whoever blasphemes against the Holy Spirit will never be forgiven, but is guilty of an eternal sin” 30 (because they said, “He has an unclean spirit”). NET
The easy answer to this unforgivable sin is to say, as many do, that it applies to “them” (meaning the Jews) and not to “us”. When read in the context of some of the earlier verses in Mark, such as His withdrawal in 3:7 and the talk of new wineskins versus old wineskins, the argument is that Jesus is effectively shutting the door on Judaism. This can lead to a form of supersessionism (replacement theology) which essentially holds that because the Jews did not keep their covenant with God, He replaces Israel with the church. A big problem with replacement theology is that if God can change His mind because of nonperformance (or for that matter, any reason) then the promises made to us aren’t guaranteed nor as secure as we have believed.
Romans 11:1 I ask then, has God rejected his people? Absolutely not. For I also am an Israelite, a descendant of Abraham, of the tribe of Benjamin……11 I ask then, did they stumble that they might fall? Absolutely not. But by their fall salvation has come to the non-Jews, to provoke them to jealousy…. 24For if you were cut out of that which is by nature a wild olive tree, and were grafted contrary to nature into a good olive tree, how much more will these, which are the natural branches, be grafted into their own olive tree? 25 For I do not desire you to be ignorant, brothers, of this mystery, so that you won’t be wise in your own conceits, that a partial hardening has happened to Israel, until the fullness of the non-Jews has come in, 26 and so all Israel will be saved. NHEB
The coming of Christ has changed the means of salvation as it now comes through faith in Him rather than through works of the law. So in this sense, the new covenant supersedes the old covenant. On the other hand, Paul uses Abraham as an example of justification by faith and tells King Agrippa in Acts 29 that Christ fulfills what Moses and the prophets said was going to happen.
Acts 26:19 “Therefore, King Agrippa, I was not disobedient to the heavenly vision, 20 but I declared to those in Damascus first, and then to those in Jerusalem and in all Judea, and to the Gentiles, that they should repent and turn to God, performing deeds consistent with repentance. 21 For this reason the Jews seized me in the temple courts and were trying to kill me. 22 I have experienced help from God to this day, and so I stand testifying to both small and great, saying nothing except what the prophets and Moses said was going to happen: 23 that the Christ was to suffer and be the first to rise from the dead, to proclaim light both to our people and to the Gentiles.” NET
While much has been written about supersessionism and replacement theology, our concern at the moment is about unforgivable sin. There is an unforgivable sin and whatever it is, these guys just did it. A good starting point is with the word translated as “blasphemes” (https://www.biblehub.com/greek/987.htm). The word is used in a great many Bible verses and typically points to speaking evil against or to speak profanely of sacred things. Revile or reviled is another meaning of the word.
Jesus speaks the Word and is the Word but along with this is a witness via signs, wonders and miracles all of which point to who He is. John the Baptist has doubts while in prison and sends people to Jesus to verify that He really is the Messiah. Jesus instructs them to tell John all of what He is doing as that will provide all the proof he needs. From Christ’s point of view, the scribes have the Scriptures and know all about the Messiah so it should be clear to them just who He is. Instead they have rejected the inner work of the Holy Spirit. (For what it is worth, the Pharisees didn’t participate in the baptisms for the forgiveness of sins that John performed in the Jordan River). Not only have they rejected this inner work, they have considered it as coming from the evil one.
1John 5: 5 Now who is the person who has conquered the world except the one who believes that Jesus is the Son of God? 6 Jesus Christ is the one who came by water and blood – not by the water only, but by the water and the blood. And the Spirit is the one who testifies, because the Spirit is the truth. 7 For there are three that testify, 8 the Spirit and the water and the blood, and these three are in agreement. 9 If we accept the testimony of men, the testimony of God is greater, because this is the testimony of God that he has testified concerning his Son. 10 (The one who believes in the Son of God has the testimony in himself; the one who does not believe God has made him a liar, because he has not believed in the testimony that God has testified concerning his Son.) 11 And this is the testimony: God has given us eternal life, and this life is in his Son. 12 The one who has the Son has this eternal life; the one who does not have the Son of God does not have this eternal life. NET
Water can refer to either baptism or as part of a physical birth. Here it is the witness of the water when Jesus is baptized where God acknowledges Him as His Son. Although hearts become hardened slowly, a time comes where, as C.S. Lewis says, “we will look to God and say Thy will be done or He will look at us and say thy will be done.” A point of no return. It isn’t that God can’t forgive but rather it is people who don’t want (or feel that they need) to be forgiven. Chapter 3 of John’s Gospel tells us that those who have rejected Christ have been already judged. St. Isaac of Syria (from Nineveh, no less) made the argument that the glory of God in Heaven and the flames in Hell are exactly the same. The difference depends on where you stand in relation to God.
Our focus has been on the unforgivable sin but don’t lose sight of the fact that verse 28 begins by telling us that all sins (except the one) can be forgiven. The church’s traditional answer has been “if you are worried about committing this sin, you haven’t”. While this isn’t a Biblical answer, Paul tells us in 1Corinthians 12:3 that no one speaking under the influence of the Spirit can ever say that Jesus is accursed. This influence sets us apart from the Pharisees and the Jewish leaders in these verses.
Mark 3:31 By this time His mother and His brothers arrive, and standing outside they send a message to Him to call Him. 32 Now a crowd was sitting round Him; so they tell Him, “Your mother and your brothers and sisters are outside, inquiring for you.” 33 “Who are my mother and my brothers?” He replied. 34 And, fixing His eyes on the people who were sitting round Him in a circle, He said, 35 “Here are my mother and my brothers. For wherever there is one who has been obedient to God, there is my brother—my sister—and my mother.” WEY
His family are not believers but they have reappeared (this story is also in Matthew and Luke). In Luke we are told about Mary’s vision so it is hard to imagine her plotting against Jesus. It is easier to imagine that she is filled with the motherly concern that He is going to be killed. Consider this apparent rejection of His family along with what happens later on the cross. Normally James would take over care of Mary upon the death of Jesus but He assigns John instead as James is not a believer at this point.
Like Jesus, the church has historically pointed to us as family and looks at all of us as brothers and sisters. Even today, there are churches where the members refer to each other as “Brother” or “Sister” followed by the person’s name. (This applies to the ministers of those churches, as well).