Whoever blasphemes against the Holy Spirit is guilty of an everlasting sin and will never be forgiven. So it is true. All sins are not the same. Catholic teaching distinguishes between venial and mortal sins, the mortal ones being the more grave ones. Mortal sins are classified as such because of the harm that they do: they kill charity that ought to exist amongst people. Mortal sin impedes an individual from being charitable towards others. While Jesus is obviously not referring to our classification of sins, he too understands that there are sins that are graver than others. Jesus talks about a grave sin being that of blaspheming against the Holy Spirit. How does one blaspheme against the Holy Spirit?
Jesus had just healed a demoniac and driven out an evil spirit. In the crowd as Jesus did this were some scribes. And as has been the case with other religious leaders, they refuse to acknowledge the good that Jesus has done. However, these scribes take the fight a little further: they attribute Jesus' healing powers to Beelzebul, the prince of demons. For reasons that are known to them alone, they refuse to see the hand of God in what Jesus has done. It is this attribution that has Jesus worked up. It would be tolerable if they refused to believe in Jesus (cf. John 10:38), but it is something else when they attribute the good work God has accomplished through him to the evil spirit.
Good cannot come out of evil. Good only comes from good. And we know that only God is good (cf. Mark 10:18). Healing of a demoniac is good, and as such attributing the healing to the evil spirit is the same as being spiteful of God. It is a sin that can never be forgiven. Are we spared of this charge? Not likely. How many times, because of jealousy, have we refused to acknowledge our brothers and sisters' accomplishment? There are times that we too have refused to see the hand of God working in our brothers and sisters. Sometimes we even go as far as belittling the good that God has worked in their lives. Behaving in such a manner equals attributing God’s good work to the evil one.