Shaykh Ṣālih al-Fawzān (may Allāh preserve him) stated:
“If the one being refuted is from Ahl al-Sunnah wal-Jamāʿah, then the refutation is conducted with manners. He is shown his errors that are related to the matters of fiqh, deduction and ijtihād. So we can say, ‘so-and-so erred and what is correct is such-and-such, and here is the proof, may Allāh forgive him, and that was his own ijtihād.’ This is how it is done just as the refutations that would take place between the jurists of the Four Madh´habs and other than them.
So, this person is not vilified with respect to his knowledge if he is from Ahl al-Sunnah wal-Jamāʿah. And Ahl al-Sunnah wal-Jamāʿah are not infallible – they can err, and the proof can pass them by, or they can miss the mark in deduction. So, we do not remain silent concerning the mistake, rather we clarify it while excusing his mistake due to the saying of the Prophet (صلى الله عليه وسلم): “When a judge judges in a matter, he made ijtihād, and he was correct, for him there are two rewards. And if he judges in a matter, he made ijtihād but then he erred, for him there is one reward.” (Al-Bukhārī, no. 6919, Muslim, no. 1716) This is related to matter of fiqh.
As for affairs of creed (al-ʿaqīdah), then it is not allowed for us to praise the misguided ones, the opposers of Ahl al-Sunnah wal-Jamāʿah from the Muʿtazilah, the Jahmiyyah, the Zanādiqah (heretics), the Malāhidah (apostates), and the people of deviation in these times, and how numerous they are!”
(Reference: al-Ajwibah al-Mufīdah ʿan As’ilah al-Manāhij al-Jadīdah, pp. 32-33, Dār al-Ma’thūr)
Read an example here of how differing should be dealt with, bārakallāhu fīkum.