Apostasy, the unforgivable sin (i.e. blasphemy of the Holy Spirit, falling away) is the result of refusing to obey the official prescription of repentance/justice issued by Christ’s church (through her elders/judicial committee). Simply put, it is unrepentance in re: to the church.
1.1. Rejecting the authority/power of the Holy Spirit is an unforgivable sin/apostasy (Mat 12:22-32):
1.1.1. Jesus is accused by the Pharisees of performing exorcisms via the authority/power of Satan (22-24).
1.1.2. Jesus validates His authority/power as coming from the Holy Spirit (28 – “by the Spirit of God”) based on:
126.96.36.199. The non-sequitur caused by exorcism being the result of Satanic authority/power (25-26)
188.8.131.52. The exorcisms performed by the Pharisees’ sons (27)
184.108.40.206. The fact that He is (by the exorcisms) binding and plundering Satan (29).
1.1.3. Jesus makes it clear that those not submitting to Him:
220.127.116.11. Is both His enemy and an enemy of the kingdom (30 w/28)
18.104.22.168. Can be forgiven (31-32 – “every sin of blasphemy will be forgiven people…And whoever speaks a word against the Son of Man will be forgiven” = When people sin against Christ through disobedience to His Word, there is provision for forgiveness, 1Jo 2:1-2).
1.1.4. However, refusing to repent and recognize the authority of the Holy Spirit (by which Jesus was operating) is the unforgivable sin (31-32 – “blasphemy against the Spirit will not be forgiven…whoever speaks against the Holy Spirit will not be forgiven, either in this age or the age to come.”).
1.2. The covenant community/church possesses the authority/power of the Holy Spirit (Joh 20:21-23):
1.2.1. Jesus deputizes the elders of the New Covenant community/church with the same authority/power (of the Holy Spirit) given to Him by the Father (21-22; Mat 3:16)
1.2.2. This is what gives the church’s elders the exclusive ability to interpret God’s Word, establish doctrine and most especially, to determine who is (and is not) repentant and receiving God’s forgiveness/salvation (23; e.g. Act 15:1-28).
1.3. As such, refusing to obey the prescription of repentance/justice issued by the church (as approved by her elders) is how someone today would be guilty of committing the unforgivable sin/apostasy (Mat 18:15-20):
1.3.1. Jesus re-establishes OC jurisprudence (incl. a judicial council) as the primary means to seeking repentance/justice within the covenant community (15-17 w/Deu 16:18-20, 17:1-13; 1Co 6:1-8, 14:34; consider also Act 15:22 w/ Num 35:12, 24-25).
1.3.2. This includes the consequence of apostasy for those who refuse to obey the prescription of repentance/justice issued by the church or church elders (Deu 17:8-13 w/Num 15:30-31 w/17 –“If he refuses to listen even to the church, let him be to you as a Gentile and a tax collector.”)
1.3.3. Jesus reaffirms the authority/power of the church/church elders in such decisions/rulings (18; Mat 16:19)
1.3.4. Jesus makes it clear that such ruling/decisions have both the backing of the Father and Himself (as High Priest) (19-20; 1Co 5:3-5).
1.4. This is what some Jewish Christians were in danger of doing in the first century (Heb 6:1-12, 10:19-30):
1.4.1. The larger context of both passages is the superiority and permanence of Jesus Christ and His sacrifice, the New Covenant and church versus the Levitical priesthood, sacrificial system, Old Covenant and church in matters of salvation (1-5 for 6:1-12; 7-10:18 for 10:19-30).
1.4.2. In light of this larger context, (Heb 6:1-12 and 10:19-30) share the following 2 responses:
22.214.171.124. The necessity of growth, faithfulness and perseverance by those who have entered into the New Covenant community (i.e. the church) (6:1-3, 10:19-25)
126.96.36.199. The consequence of apostasy afforded to those who reject the call to repentance issued from the New covenant community – i.e. who “fall away”, or “go on sinning deliberately” (6:4-12, 10:26-30).
1.4.3. That there has indeed been a call to repentance issued by the covenant community is made clear once we consider the authors’ choice of words again in (6:4), “it is impossible to restore them to repentance”. You cannot be restored to something you were never in the process of originally doing. Therefore, by the author words we can be certain that these individuals were in the process of repenting when they fell away.
1.4.4. That it is indeed the authority of the church in these matters that is being rejected – or these individuals are in danger of falling away from (or sinning deliberately against), is affirmed by:
188.8.131.52. The immediate context (it is again about remaining in the covenant community – see 184.108.40.206.),
220.127.116.11. The church is the only one who can declare someone apostate (“fallen away”, “sinning deliberately”) (see again Mat 16:19)
18.104.22.168. This is the locative context of (6:1-2, 10:28 = Deu 16-17; Notice also in 6:1,3,11: the author speaks in the 1st person plural implying that the warning is coming from church/elders rather than a concerned individual)
22.214.171.124. This is how the author ends the book: w/ a warning regarding the authority/power of the New Covenant community (12:18-29).
It is important to note that biblical repentance always requires doing justice in re: to all sin – not simply the sin that has brought about the call (to repent). In other words, a person is only viewed as repentant from any sin when they commit to turn from every sin (Jam 2:8-11 = you w/b guilty of the 2nd greatest commandment unless you do justice to your brother in re: to all God’s commands; e.g. Luk 19:1-10). As such, repentance/doing justice in re: to all sin is implied in the church’s prescription regarding a particular sin and therefore also what it means to obey their ruling.