Okay, let's look at this a minute. Within Christianity I would say their are five primary theologies:
1. Catholic/Orthodox
2. Lutheran
3. Calvinist
4. Anglican
5. Arminian/Baptist
Now, I know we could argue about this list and I am certainly open to different categorizations, but I think most of us can agree that there are a few distinct and conflicting theologies within Christianity. So, the question becomes, since the Holy Spirit CANNOT LIE, which ones are wrong?
Rejecting Christ is the only truly mortal sin.
Actually, Christ said that the ONLY sin that could not be forgiven is blasphemy of the Spirit. But what does that actually mean? I think a case could be made that saying a theology comes from the Holy Spirit when in fact it was just made up could fall into the category of speaking against the Holy Spirit.
It is simple, and can be done by anyone. But it requires repentance for your sins. Which implies that you will try to overcome them.
If you do not, perhaps you just made an emotional commitment, and I would say were not truly saved to begin with, regardless of you has baptized you.
Pentecostals, it seems to me, make this error, often, but I have known many that I have no doubt are true Christians.
Everything else is, well, important, but will not cost you your salvation if it is mistaken. But to me, having to , I don't know, do confession to a priest, eat bread that you believe is actual flesh, finger beads, etc. smacks of magic far more than checking the Bible our for yourself, and following where that leads you. No offense. Unlike some, I do not dispute the Christianity of the RCC.
As for your second question, I believe that the blasphemy against the Holy Spirit is precisely the final rejection of Christ.
In fact, I may be mistaken, I think this is probably the RCC's position, too. On one website by a RCC priest, the question of "once saved, always saved" came up, a good Baptist position. Naturally the good Father was having none of it.
What he said, and is in fact the correct interpretation of said doctrine, is that those who hold to that position say that if you have "lost" your salvation, then you didn't have it to begin with.
Now, I could give the arguments and verses to support this position, but that isn't my point. What the priest did say was, "But our Baptist friends have it right in its essentials. The unforgivable sin is to die with a final rejection of Christ." Or words to that effect. As I said, I do not know the official Vatican position on that.