I prefer the motto “Nothing is Sacred.”
I do think personal insults & abuse are below the line.
However there is a difference between Islamophobia (abusing people because they are Muslim) and criticising Islam.
To apply this to the Bahá’í faith; I obviously would be sensitive about my Prophet but at the same time criticising my faith doesn’t necessarily mean that I am being criticised.
as a good rule of thumb we should try and avoid gratuitous abuse in any scenario but intellectual questions are necessary.