Today, we will consider an alternative interpretation. I personally believe that the religious leaders did commit the unpardonable sin in this context. They had just witnessed an amazing miracle, and they had rejected it! It is important to remember what Jesus stated back in Matthew 12:28. He said that He was casting out demons by "the Spirit of God"; that is, by the power of the Holy Spirit. But, the Pharisees rejected this amazing miracle because they had allowed their hearts to become so hardened against God's truth and the power being manifested through Jesus. How did they reject this miracle? They rejected it by attributing the power of the Holy Spirit to Satan! It is reasonable to believe that doing such is blasphemy against the Holy Spirit.
This interpretation becomes even more reasonable when considering Mark 3:30 (a parallel passage). This verse begins with the word "because". Jesus made the statement regarding the unpardonable sin in Mark 3:29 because the religious leaders had said - "He has an unclean spirit." Is this not the same as saying: "Jesus' power is of Satan!"? Indeed it is (cf. 3:22). It seems sensible to conclude that our Lord is saying to them - "You can't be forgiven of what you just said because you've blasphemed the Holy Spirit!"
This raises another question: Why is it that this sin is unpardonable? Is it unforgivable because God desires to hold an eternal grudge against those who make ungodly accusations against His Spirit? No. It is unforgivable because these Pharisees had totally rejected God; they had let their hearts go beyond "the point of no return." They were so hardened that even witnessing this great miracle couldn't penetrate their hearts. These Pharisees had committed a sin that could never be forgiven because their hearts were so hard and calloused that they wouldn't ever be able to repent! It is true that God does not want anyone to be lost (I Tim. 2:4), but a fundamental requirement of being forgiven is repentance (cf. Luke 13:3; II Pet. 3:9; Acts 17:30). Surely, if God is willing to forgive a person for blaspheming Jesus, He would also be willing to forgive a person for speaking against the Holy Spirit. But, the fact is, Jesus knew (John 2:25) that these Pharisees were so hard that it would be impossible for them to ever repent (cf. Matt. 23:33)! It would have been one thing to simply reject Jesus' teachings and speak out against Him ignorantly. However, it is an entirely different matter for one to witness the awesome miracles He worked by the power of the Holy Spirit and then revile the Spirit by affirming that the devil's power is at work! Only the most depraved soul could do such a thing! Thus, I believe the unpardonable sin is attributing the miraculous works of the Spirit to the power of Satan. This sin cannot be forgiven because no one would attribute Jesus' power to Satan unless his heart was so hard that nothing could ever penetrate it and lead him to repent.
If this conclusion is correct, it would be impossible for anyone to commit this specific sin today since the age of the miraculous gifts of the Spirit has passed. However, just because a person today might not attribute Jesus' power to Satan does not mean that such a one cannot commit sins that are unforgivable. It is certainly possible for one to develop a heart that is so hard that it becomes impossible for him to repent. And, if it is impossible for one to turn from his sins, then it is impossible for God to forgive him! Any sin, under these circumstances, will become unforgivable, whether it has anything to do with blasphemy or not.