Disagreement in the fundamentals leads to disagreement in the branches, such as the prohibition of a Hanafi marrying a Shafi'i woman because, in their view, she doubts their faith.

The Shaykh : This disagreement concerning creed, that is, in the fundamentals, has entered into the branches. This is because it is mentioned in some of the books of the Hanafis that one of their great scholars was asked: Is it permissible for a Hanafi to marry a Shafi'i woman? That is, a woman who is a Shafi'i. He answered in the negative, saying, "It is not permissible." Why? He said, "Because the Shafi'is doubt their faith, and whoever doubts his faith has committed disbelief." This statement is mentioned in more than one book of the Hanafis, and what comes to my mind now is "Al-Bahr al-Ra'iq Sharh Kanz al-Daqaiq" by Ibn Najim al-Misri. He mentioned in the chapter on disliked matters among the causes of declaring someone a disbeliever that a Muslim should say, "I am a believer, if Allah wills." And they derived from this what you have heard of the statement that a Hanafi is not permitted to marry a Muslim woman because she says, "I am a believer, if Allah wills." What is notable here is the ignoring of the creed of others and the unjust bias against them, and this is one of the effects of sectarian fanaticism. This is because when the Hanafis issue this ruling that whoever says, "I am a believer, if Allah wills," is a disbeliever, they know that this speaker does not trust in the faith in his heart, but rather doubts the action that comes from his limbs. So the doubt of the Shafi'is is not directed at the faith as defined by the Hanafis, which is speech and affirmation. No, they do not doubt this. Rather, they doubt the effects of faith that follow from strong or weak faith. So if one of them says, "I am a believer, if Allah wills," he does not mean, "Do you doubt the faith in your heart in Allah and His Messengers and everything that came from Allah?" The answer is no. Rather, I doubt my righteous deeds, that I am shortcoming, that I am such and such, and so on. So here there are two things, one more severe than the other, and both have passed. The first is the statement that faith does not increase or decrease, contrary to the texts of the Book and the Sunnah. The second is ignoring this creed held by the other party of Muslims and ruling upon them with takfir (declaring them disbelievers) because they differed from them in their school of jurisprudence. But I also want to ask: Did the matter stop at this point of saying that it is not permissible for a Muslim, or for a Hanafi, to marry a Shafi'i because the Shafi'is doubt their faith? You heard that a fatwa was issued stating that it is not permissible, and that whoever says, "I am a believer, if Allah wills," is a disbeliever. A fatwa was later issued by a famous scholar among the Hanafi scholars that agrees with the statement of declaring a disbeliever whoever says, "I am a believer, if Allah wills," but it differs from that fatwa which did not permit a Hanafi to marry a Shafi'i. That scholar is known as Abu al-Sa'ud, the Mufti of the Two Heavyweights, who has the well-known book of tafsir, "Tafsir Abu al-Sa'ud." This Hanafi scholar and exegete was asked the same question: Is it permissible for a Hanafi to marry a Shafi'i? He answered by saying, "It is permissible." So he was lenient, and his answer was closer to the truth than the statement of the one who prohibited it. That one said, "It is not permissible," while this one said that it is permissible for a Hanafi to marry a Shafi'i. But if you look at his reasoning for his ruling of permissibility, you will know that the problem is very serious. Because with that reasoning, he confirms the disbelief of whoever says, "I am a believer, if Allah wills." Because when he issued the fatwa of permissibility, he said, "It is permissible for a Hanafi to marry a Shafi'i, treating her as the People of the Book." That is, just as it is permissible for a Muslim to marry a Jewish or Christian woman, it is permissible for this Hanafi to marry a Shafi'i because she is of the People of the Book; she believes in the Qur'an and the hadith of the Messenger (peace and blessings be upon him). The point of this sequential matter, involving creed, jurisprudence, and reality, is that disagreement in the fundamentals... The point is that disagreement in the fundamentals leads to disagreement in the branches, and that disagreement in the branches is like disagreement in the fundamentals in that it leads to Muslims declaring each other disbelievers. Therefore, the point of all this is that a Muslim should not be lenient and say, "There is no harm in disagreement, because disagreement is only in the branches, not in the fundamentals," because this statement contradicts reality in both its parts. That is, disagreement is a reality in the branches just as it is a reality in the fundamentals, and some of the branches are considered like disagreement in the fundamentals, meaning that it leads to takfir. Then there is another difference that we must know, which is that those who say and divide disagreement into two categories: one category is in the branches, so disagreement is permissible in it and the one who errs in it is excused; and the other category is disagreement in the fundamentals, so the one who errs in it is not excused and becomes a disbeliever, everyone who falls into contradicting the Shari'ah in this category, which is the fundamentals, that is, creed. Is this distinction also correct and in accordance with the Qur'an and the Sunnah? That is, that there is no harm in disagreement in the branches, so this is not the case, and that the harm is the greatest harm in disagreement in the fundamentals, and that a Muslim cannot be excused if he falls into an error in which he contradicts the fundamentals? The answer is that this is not correct, and the first is not correct either. That is, just as it is not permissible for a Muslim to intentionally contradict the Book and the Sunnah in the branches, so it is not permissible for a Muslim to intentionally contradict in creed, in faith, in the matters of the Book and the Sunnah. This is the exact opposite of that, there is no difference between one who intentionally contradicts the text in the Book and the Sunnah in the branches, and there is no difference between one who intentionally contradicts the Qur'an and the Sunnah in the fundamentals. Rather, this person is held accountable by everyone, a fortiori. But the reverse is also the truth. That is, just as one who contradicts the truth in the branches from what came in the Book and the Sunnah is not held accountable for an excuse, whatever that excuse may be, so one who contradicts the Shari'ah or the text from the Book or the Sunnah in the fundamentals is not held accountable, there is no difference. The important thing, or the criterion of the ruling, is the intention to contradict or the lack of intention to contradict. So one who intentionally contradicts the Book and the Sunnah in the branches is like the one who intentionally contradicts in the fundamentals. And one who does not intentionally contradict the Book and the Sunnah in the fundamentals is like the one who does not intentionally contradict in the branches. This distinction also has no share of truth.