Is it permissible to utter religious discourse in colloquial Arabic, specifically by pronouncing the letter "ب" (b) with a kasra in the word "ربنا" (Rabbana - Our Lord) despite it being obligatory to pronounce it with a damma, in normal conversation and not during prayer?

It is not permissible to recite the Qur’an or Hadith except in the manner in which they were revealed, and there is no difference in this regard between whether it is during prayer or outside of it. As for speech other than the Qur’an and Hadith, there is no objection to speaking it in the vernacular, although it is better to speak it in classical Arabic.