Humans like to be part of exclusive clubs. And they like putting people outside of their club. The Quran says those who say Allah is one of three are kafirun. As Muslims should we arrogantly look upon all Trinitarian Christians as kafirun who are all bound for hell? BTW, those who reject the Prophet hood of Muhammad are also kafirun, not just Trinitarians. When speaking to Christians is it always a good idea to emphasise their kufr or might it be better to put the shared and common values forward? There are subtleties here. Arrogance puts people off. Not everyone has had access to the message of Islam, some may have a very distorted picture....so we need to be wise and vast in our mercy and humility, you never know this might win people over more than a harsh exclusivist invitation to Islam.