The one sin that Jesus Christ himself warned us was a 'death-dealer' was the sin of blasphemy against the Holy Spirit:
"He who is not with me is against me, and he who does not gather with me scatters. And so I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come." (The Gospel of St. Matthew 12:30-32,NIV)
Every other sin, irregardless of what that sin may be, is equally sinful in God's eyes. Only blasphemy against the Holy Spirt is identified as 'sin to the max'. Which leaves us with two questions, namely, why? and What? Why is blasphemy against the Holy Spirit especially sinful, and what exactly constitutes blasphemy against the Holy Spirit?
Personally, I believe that it's due to the strong probability that when we refer to the Holy Spirit, we are not referring to some force that emanates from a 'big guy' on a throne as if it were a laser beam. Rather, we are actually referring to the very essence of God himself. The words 'Son' and 'Father' are titles, not people. The same person can be a son, a father, a brother, an uncle, a cousin, or a myriad other things, while still being only one person.
But whenever the Spirit is referred to, it is very specific. There we are no longer encountering a title, but instead an actual Being. In John 4:24 Jesus himself identified God as being a Spirit. I don't think his stating that God is Spirit, and his stating that we are to 'walk softly' when referring to the Holy Spirit, is coincidental. So we are strictly ordered to deal respectfully with the subject of the Holy Spirit.
And what is blasphemy against the Holy Spirit? It is not doubt, though some would attempt to convince you otherwise. Doubt is doubt, honestly recognized and honestly expressed. God will help you deal with that, without anger and without chastisement.
Blasphemy falls within these borders:
Jesus himself had just performed a healing. It was obviously a genuine happening, but rather than give God the credit due him, there were those who claimed that Jesus was actually healing people and casting out demons on the orders of Beelzebub, the prince of demons.
There are many that get up to preach the word while honestly depending on the Holy Spirit to give them a hand. But there are others who invoke his name in a blatant attempt to intimidate and overpower any and all who are unwilling to accept them as their spiritual leaders. They claim to be uttering the message of the Holy Spirit, but they do it out of a selfish desire for more-and-more power for themselves. Examples of this are, "If you truly love Jesus, you'll ..."; "To disobey me is to disobey God himself"; "The Spirit has sent me to tell you ..."; "The Spirit speaks through me, so you must obey what I tell you to do". To say any of these statements, or others akin to them, while knowing at the time of their utterance that they are being used as a manipulative device rather than their being an honest communique from God, is blasphemy.
We do have a code of conduct, which can be found in Galatians 5:16-26. There St. Paul stated clearly how we are supposed to behave. But the man who wrote that passage also wrote this one:
We know the law is spiritual, but I am unspiritual, sold as a slave to sin. I do not understand what I do. For what I want to do I do not do, but what I hate I do. And I do what I do not want to do. I agree that the law is good. As it is, it is no longer I myself that do it, but it is sin living in me. I know that nothing good lives in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For what I do is not the good I want to do; no, the evil I do not want to do - this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it.
So I find this law at work: When I want to do good, evil is right there with me. For in my inner being, I delight in God's law; but I see another law at work in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members. What a wretched man I am! Who will rescue me from this body of death? Thanks be to God - through Jesus Christ our Lord. (Romans 7:14-25a,NIV)
Do your best, but don't expect perfection. Instead, trust God to see you through, in the same way as a child trusts its parent to help and guide it, even when the child doesn't know exactly where the parent is taking the child.