{To know the mysteries} (gn(936e)ai ta must(8872)ia). Second
aorist active infinitive of gin(9373)k(935c). The word must(8872)ion is
from must(8873), one initiated, and that from mue(935c) (mu(935c)), to
close or shut (Latin, _mutus_). The mystery-religions of the east
had all sorts of secrets and signs as secret societies do today.
But those initiated knew them. So the disciples have been
initiated into the secrets of the kingdom of heaven. Paul will
use it freely of the mystery once hidden, but now revealed, now
made known in Christ ( Ro 16:25  1Co 2:7 , etc.). In  Php 4:12 
Paul says: "I have learned the secret or been initiated"
(memu(886d)ai). So Jesus here explains that his parables are open
to the disciples, but shut to the Pharisees with their hostile
minds. In the Gospels must(8872)ion is used only here and in the
parallel passages ( Mr 4:11  Lu 8:10 ).