The Mufassir first and foremost must possess a correct belief in Islam for his or her Tafseer to be pure and free from heresy, or gross errors. Sincerely believing in Islam does not automatically mean that one who does so has correct belief in Islam. Ignorance of what constitutes correct belief in Islam will almost certainly lead the Mufassir into incorrect explanations. Such an individual will be unable to distinguish between a correct interpretation and an incorrect one. Consequently, he or she will have to rely on their personal judgement, which would be impaired due to their ignorance.
Correct belief is also non-sectarian.
The Mufassir does not approach the Qur'aan with pre-conceived ideas and notions which he or she wishes to find support for in the Qur'aan. Such an approach invariably leads to misinterpretations and sectarian explanations. All honest attempts at Tafseer mush begin with the Tafseer of the Qur'aan by the Qur'aan itself. What remains unexplained must then be sought in the Sunnah. If the Tafseer still can not be found, the explanations of Sahaabah and their students must then be turned to.