Mt 12:31f “People will be forgiven for every sin and blasphemy, but the blasphemy against the Spirit will not be forgiven. 12:32 Whoever speaks a word against the Son of Man will be forgiven. But whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.”

Mk 3:29 “Whoever blasphemes against the Holy Spirit will never be forgiven, but is guilty of an eternal sin”

Lk 12:10 “The person who blasphemes against the Holy Spirit will not be forgiven.”

Carson remarks that this saying is all the more remarkable, given the emphasis throughout Scripture on God’s grace and mercy (e.g., Ps 130:3–4; Isa 1:18; Mic 7:19; 1Jn 1:7).

As Murray Harris (Navigating Tough Texts, p26f) explains, these sayings appear to have been spoken on different occasions.

In Mark, the context is that Jesus was being accused of having an ‘impure’ spirit, and was under the control of Beelzebul, the prince of demons.  It was an unforgivable sin against the Holy Spirit, Jesus says, to attribute that Spirit’s work to Satan.

Mounce:

‘Jesus is saying to his antagonists that to attribute to Satan that which has been accomplished by the power and Spirit of God is to demonstrate a moral vision so distorted that there is no longer any hope of recovery. It would be possible to speak against the Son of Man and be forgiven because at that time in Jesus’ ministry there was a hiddenness about his person. Not so with the mighty works wrought by the Spirit. They were clear demonstrations that the kingdom (power and reign) of God was present in the world. Denial of this was not the result of ignorance but of a willful refusal to believe. Therefore it is unforgivable. The only sin that God is unable to forgive is the unwillingness to accept forgiveness. Thus the “unforgivable sin” is a state of moral insensitivity caused by continuous refusal to respond to the overtures of the Spirit of God.’ (Emphasis added)

Evans, similarly:

‘Jesus does not dismiss the importance of blasphemy against himself, but he recognizes that to speak against him implies that a person does not know his full identity. Through greater revelation and understanding, that deficiency can be overcome: the person can repent, and the person can then find forgiveness of sin. By yielding to the Spirit’s evidential and convicting work, a person can be led to that point. The only true “unpardonable sin” is when a person consciously, willfully rejects the operation of the Spirit bearing witness to the reality of Jesus as the Savior, and rejects the convicting power of the Spirit in his or her life.’  (Holman Apologetics Commentary)

Morris:

‘When a person takes up a position like that of the Pharisees, when, not by way of misunderstanding but through hostility to what is good, that person calls good evil and, on the other hand, makes evil his good, then that person has put himself in a state that prevents forgiveness. It is not that God refuses to forgive; it is that the person who sees good as evil and evil as good is quite unable to repent and thus to come humbly to God for forgiveness.’

(Pillar)

In Luke, the context is of apostasy, of disowning Jesus.  Morris comments:

‘Jesus has been warning his disciples against hypocrisy, reminding them that everything purportedly concealed will ultimately be revealed (Luke 12:1–3). He then admonishes them to fear God, not their adversaries (Luke 12:4–7), before continuing, “Whoever disowns me before others will be disowned before the angels of God. And everyone who speaks a word against the Son of Man will be forgiven, but the person who blasphemes against the Holy Spirit will not be forgiven” (Luke 12:9–10). That is, to deny on earth that one belongs to Jesus, the Son of Man (v. 10a), has eternal consequences before the heavenly tribunal. If verse 10b looks back to verse 9, to disown Jesus—to become apostate—is to blaspheme against the Holy Spirit.’

Mark’s and Luke’s records are complementary, not contradictory.  As Harris says:

‘To have an attitude implacably opposed to God (Mark) or to commit apostasy (Luke) are simply two expressions of the permanent and irremediable rejection either of God himself (Mark) or of a faith in God once held (Luke), in spite of the gracious overtures of the Holy Spirit.’

Harris continues:

‘Did Peter commit the unforgivable sin when he disowned Jesus three times (Luke 22:54–62)? No, because he “turned back” and strengthened his brothers (Luke 22:32).

‘What of Ananias and Sapphira, apparently believers, who lied to the Holy Spirit and conspired to test the Spirit of the Lord (Acts 5:3, 9)? All we know for certain is that there was immediate divine judgment on them both for their conspiracy of deceit (Acts 5:5, 10), but we cannot know their eternal destiny.

‘It is said of Simon the sorcerer the he “believed and was baptized” (Acts 8:13). But when he tried to bribe Peter and John so that he could (magically?) convey the gift of the Holy Spirit, Peter responded with the rebuke, “May your money perish with you!… your heart is not right before God” (Acts 8:20–2). Simon’s feeble response to Peter’s directive to repent (Acts 8:22, 24) suggests he remained “captive to sin” (Acts 8:23) as a hardened unbeliever who lacked God’s forgiveness.

‘Finally, was Paul guilty of the “eternal sin” because of his systematic persecution of Christians (Acts 9:1) that even involved efforts to make them blaspheme (Acts 26:11)? No, because he “acted in ignorance and unbelief” (1 Tim 1:13) and embraced the light of the gospel when it confronted him (Acts 9:3–9; 2 Cor 4:6).’  (Formatting added)

Hendriksen, similarly:

As to other sins, no matter how grievous or gruesome, there is pardon for them.  There is forgiveness

• • for David’s sin of adultery, dishonesty, and murder (2 Sam 12:13; Ps. 51; cf. Ps. 32);
  • for the “many” sins of the woman of Luke 7;
  • for the prodigal son’s “riotous living” (Lk 15:13, Lk 15:21-24; );
  • for Simon Peter’s triple denial accompanied by profanity (Mt 26:74-75; Lk 22:31-32; Jn 18:15-18, Jn 18:25-27; Jn 21:15-17); and
  • for Paul’s pre-conversion merciless persecution of Christians (Act 9:1; Act 22:4; Act 26:9-11; 1 Cor 15:9; Eph 3:8; Php 3:6).

(Formatting added)

Harris concludes:

‘Blasphemy against the Holy Spirit is unforgivable, not because God is unwilling to forgive, but because the repentance that is the necessary precondition for God’s forgiveness is absent. The heart has become so hardened that no need for repentance is recognized, and so no request for forgiveness is offered. Strangely, to have a fear that you have committed the unpardonable sin is evidence that you have not done so, for those who have are unaware of their sin or unconcerned about it.’