Islamic extremism is founded on a dualist worldview: the realm of truth and the sacred (dar al Islam) set in opposition to the realm of falsehood, chaos and war (dar al harb). An ideology of contestation underpins Islamist radicalisation. And Islamic political thought is inherently theological; any response to the political ideology that arises from the dualist worldview must necessarily address allied theological perspectives and presuppositions. In October 2007 an ‘Open Letter and Call from Muslim Religious Leaders’ emanating from Jordan, entitled A Common Word Between Us and You, was issued to the Christian Church worldwide. What is at the heart of this ‘call’? What does it suggest with regard to an Islamic theological counter to dualism? What might it portend for the future of relations with Islam? Does it signal a new era for theological dialogue with Islam and co-operative conjoining in the wider struggle against radicalisation and extremism? This paper will introduce the letter, review some representative responses to it thus far, note some issues and challenges that are raised and, by way of conclusion, offer a perspective on containing ideological extremism through interfaith theological dialogue. Can the dialogue option enable the addressing of theological factors inherent in the ideological underpinnings of Islamic extremism, thereby acting to contain it?