[a. F. catholique (13th c. in Littré) ad. late L. catholic-us, a. Gr. καθολικός general, universal, f. καθόλου (i.e. καθ' ὅλου ) on the whole, in general, as a whole, generally, universally, f. κατά concerning, in respect of, according to + ὅλος whole. (If immed. derived from L. or Gr., the Eng. word would, according to the regular analogy of words in -IC, have been accented caˈtholic).]
In other words, a word used to mean "broad in sympathies, tastes or interests" is derived from a phrase meaning "in general, on the whole".
Greek ὅλος "whole" is from Proto-Indo-European *solh₂- "whole". This became Latin sollus "entire", which perhaps combined with citus "set in motion" (from *kei-) to form sollicitus "thoroughly moved, agitated, disturbed" and sollicitāre "to vex". sollicitāre became French soucier "to care" and insouciant "careless".