The Knights of the Fold are a martial order. They believe in one deity, Obelis, who they consider to be "the one true God", and forcefully (and violently) converts others to join their cause. They are considered sworn enemies of the order of Aeol Drias.
Chen is a member of the Fold, having faced the same brutal conversion method in his youth, now driven by fanaticism and exercising the same practices in the field.
Grimstroke implies that the order is made of charlatans, given their unreasonable fanaticism and bloody conversions.