‘Abdullah b. Zayd Al-Numayrî reports that Al-Hasan Al-Basrî said:

They (the heretics) were destroyed by their inability in Arabic (al-‘ujmah).

Al-Bukhârî, Al-Târîkh Al-Kabîr Vol.5 p99.

And Imâm Al-Shâfi’î said:

People didn’t become ignorant and didn’t differ amongst themselves except because they left Arabic and leaned towards the language of Aristotle.

Quoted by Al-Suyûtî in Sawn Al-Mantiq p15. He said on p22:

I have found Salaf before Al- Shâfi’î indicate what he did: that the cause of heresy (al-ibtidâ’) is ignorance of Arabic language.