Claude Frollo, who adopted Quasimodo,studied all the branches of theology, including "decretals".
Source: Webster's Revised Unabridged Dictionary (1913)

Decretal \De*cre"tal\, a. [L. decretalis, fr. decretum. See
Appertaining to a decree; containing a decree; as, a decretal
epistle. --Ayliffe.
Decretal \De*cre"tal\, n. [LL. decretale, neut. of L.
decretalis. See Decretal, a.]
1. (R. C. Ch.) An authoritative order or decree; especially,
a letter of the pope, determining some point or question
in ecclesiastical law. The decretals form the second part
of the canon law.

2. (Canon Law) The collection of ecclesiastical decrees and
decisions made, by order of Gregory IX., in 1234, by St.
Raymond of Pennafort.