Belief in Tahreef to a degree can be kufr.

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I want to say,  reason and logic, foundational before believing in divine books, before Messengerhood, is the belief in guidance. Than with little reflection, it becomes clear guidance is from God.  Than with more thought, it becomes clear to the extent God will attempt to clarify his guidance and proofs.  That is where Messengerhood and Prophethood become easy to believe in. Divine books which is God speaking to humanity, as we know, has a different effect than that of humans to one another.

However, there is exception handling. To unite humanity on guidance, sealing revelations and hence Prophethood, is a must.

It is inevitable. We claim this happened through Mohammad, Christians through Jesus, etc, different religions claim they are the ending religion to be united upon.

But something in all this is clear.  Revelations were only allowed to be distorted in the past because revelations would follow up.

God didn't make an original revelation and then reveal the exact same words over and over again, but in reality, it was paraphrasing the same truths, the same proofs, the same signs....each book, that is why Quran says there is no change in his words or that Mohammad didn't come with something more or different then Messengers in the past.

Now - I think just as it is true the same proofs and reasoning that apply to Prophethood that Quran shows applies to believing in those who hold authority after Mohammad from his nation of believers,  the same exact application of why God coupled leaders with revelation in the past, proves the need of safeguarding the revelation.

That is why belief in tahreef with some recitations or what not which the truth can be found if a sincere honest person is seeking to know through Quran which one is true, is ok, but not to the degree that words have been changed or verses are missing or names of Imams deleted or what not.

Belief in Prophethood with sealing of Prophets leaves two split beliefs that becomes different in the past.

The Leadership vested in Prophets becomes vested in chosen ones like Prophet except they have no revelation coupled with them to be revealed to people which as snap shots of time and relate God's words and wisdom to the situation people are in. As Quran is for all times and place, the book is been written in a way, that is less contextual and more universal unlike books in the past, which would be far more contextual of the time and place.

So belief in Prophethood is like the moon split in half in a way,   while it was all one thing in the past, revelation and Prophets, while obviously revelations being distorted happened and was not disbelief, because other Prophets would reveal revelations in their times that clarifies the truth and God's will to that time.....

In this day and age, belief in seal of Prophets has split the belief in Prophethood in two things:

1) Protection of Quran.... This is obligatory to believe because it does away with the wisdom of coupling God's book which is something all leaders in the past were coupled with, they weren't reminders in themselves, they were reminders because they were the means to recite the book as it was meant to be.

2) Leadership of chosen non-Prophets.  This is also obligatory to believe, and Quran talks about belief in Prophets in this context, that why God doens't just bring a book from heaven to people that they can touch, without a person to explain and connect to God, is all about leadership.

Anyone who fails these two beliefs,  shows they didn't reflect on Quran as it meant to be reflected upon.   The Sealing of Prophethood of course is revealed in a Surah that heavily talks about the intergrity of the Prophet and talks about with respect to Zaid, to many wives, to his purpose of marrying them, etc, which is all controversial to this day. And it talks about it with respect to his wars, etc.... At the end, God put in this Surah, because he knows claiming to be seal of Prophets look more power hungry than saying indeed many Prophets will come confirming me.  God of course attributes purity to who he pleases and manifests it, and so if it is not God who manifests who is pure and to be trusted, than where will we go to? Who will we turn to towards God, towards clergy attributing purity to themselves or the chosen household of time?

The wisdom of succession is also a topic of Quran. That is it is one thing to believe in Leadership of chosen and revelations, but succession is also talked about, mainly, that they come in groups. And this grouping as one way split into twelve ways and branches linking back to a singel root, is something that is emphasized in Quran.

All this is related, and so the belief in Prophethood IF and ONLY IF coupled with SEALING PROPHETHOOD must come with belief in safeguarding the revelation, or else, it is purposeless and detrimental, the whole plan of sealing Prophethood is counter productive.