Whoever blasphemes against the Holy Spirit is guilty of an everlasting sin and will never be forgiven. So, it is true that some sins are more grave than others. The teaching of the Church 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 and within a human person. A mortal sin is grave because it impedes an individual from being charitable towards others. While Jesus is obviously not referring to our ‘theological’ classification of sins, he too understands that some sins are more grave 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 of him an unclean spirit. Among those who witnessed Jesus performing this sign were some scribes. And as had become the norm, following in the footsteps of the other religious leaders, the scribes refused to acknowledge the good that Jesus had done. Moreover, as if failing to see any good in what Jesus was doing was not enough, the scribes decided to up the ante in their disdain for Jesus: they attributed Jesus' healing powers to Beelzebul, the prince of demons. For reasons that were known to them alone, they refused to see the hand of God in what Jesus had done. It is this attribution that got Jesus worked up. It would have been tolerable if they simply refused to believe in Jesus (cf. John 10:38). However, it was something else when they attributed the good work that God was accomplishing through Jesus to the evil spirit. By attributing the healing to the evil spirit, they were not simply belittling Jesus: they were also giving glory to the evil one. As scribes, they ought to have known better. 
Good cannot come out of evil. Good can only come from good. And we know that only God is good (cf. Mark 10:18). Healing of a demoniac is a good thing, and as such, attributing the healing to the evil spirit is the same as being spiteful of God who is the author of all good deeds. It is a sin that can never be forgiven. 
Are we spared of this charge? How many times, because of jealousy, have we refused to acknowledge our brothers and sisters' accomplishment? There are times when 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. When we do so, we too are guilty of sinning against the Holy Spirit.