How to respond to a Cease and Desist Letter- concerning the use of a Trademarked name of an actual person who lived in the 1600's.

I am writing on behalf of an organization involved in personal and global empowerment and education, online broadcasting, and internet based and live metaphysical seminars, workshops and training. They are located in Portugal, and have been engaged in these types of activities since approximately 2006. They recently received a Cease and Desist letter from a Colorado Corporation who is a similar type of group. The dispute is over the use of a particular term in their material. Apparently this term, which is the name of an actual and very famous historical person who lived in the 1600s in France and is well known in spiritual circles and openly channeled and published by many people throughout time was Trademarked by this group sometime in 2011. This organization was not aware of this, and has actually been using the name in recorded and written material since 2006. There is no European trademark registered. How do they respond?