وَلَا تُجَادِلُوا أَهْلَ الْكِتَابِ إِلَّا بِالَّتِي هِيَ أَحْسَنُ إِلَّا الَّذِينَ ظَلَمُوا مِنْهُمْ ۖ وَقُولُوا آمَنَّا بِالَّذِي أُنزِلَ إِلَيْنَا وَأُنزِلَ إِلَيْكُمْ وَإِلَـٰهُنَا وَإِلَـٰهُكُمْ وَاحِدٌ وَنَحْنُ لَهُ مُسْلِمُونَ ﴿٤٦
And do not argue with the People of the Scripture except in a way that is best, except for those who commit injustice among them, and say, “We believe in that which has been revealed to us and revealed to you. And our God and your God is one; and we are Muslims [in submission] to Him.”
EXCERPTED FROM THE TAFSEER FROM DR. FARHAT HASHMI
In normal circumstances (e.g. in an interfaith conference) one should put ones best foot forward and deal in an Ehsan manner. (Which means Muslims should surpass the others in their conversational manners).
If emotions are running high and someone is being cruel, rude and unjust then curtail the conversation. Ehsan does not mean that one should erase the differences by manipulating the Quran and making right what is wrong.
However if the things get out of hand then say “we believe in what was revealed to us from Allah Subhanawataala and what was revealed to you.”
We have no bias against what was revealed to you (which is not necessarily what is believed by the different denominations at this time). We are Muslims and we submit to what has been revealed to us in the Quran.
A momin always errs on the side of a beautiful conversation.
When one differs with the other person: be assertive don t be aggressive.
Be firm but not obnoxious, but do not compromise facts and commands of the Quran to appear ehsan in our conversation with the people of the book.
Even in private conversation with each other (between Muslims) one should speak with ehsaan. This is good practice for those moments when emotions are high and people are defending their faith vociferously.