To what extent is the statement true that "if we were to compile the missteps of the theologiains of the People of the Sunnah and the Community and put them in a book, we would become disbelievers"?

If the intent of the statement is to warn against following the slips of scholars and acting upon them, then it is correct, because that leads to abandoning the religion. Al-Shatibi narrated the saying of Sulayman al-Taymi: "If you take every scholar's concession, all evil will gather in you." Al-Bayhaqi mentioned that Al-Awza'i said: "Whoever takes the rare opinions of scholars has left Islam." Imam Ahmad said: "If a man were to act upon the opinion of the people of Kufa regarding nabidh (fermented drink), the people of Medina regarding sama' (audition of music), and the people of Mecca regarding mut'ah (temporary marriage), he would be a fasiq (transgressor)."

However, if the intent is to disparage the scholars of Ahl al-Sunnah wal-Jama'ah, then this is a grave evil from which one must repent. Scholars are not infallible, and they may err, so they are not to be followed in their errors. However, they do not intentionally violate the Sharia, and their slips do not discredit them.

Al-Shatibi clarified that a scholar's slip must not be adopted through imitation, because it is contrary to the Sharia. However, one should not criticize the scholar or accuse them of negligence because of it.