、。
It also teaches that for serious sins, you cannot receive forgiveness without confessing to a leader.
Literal
Serious [attr-な] sin [about-については], leader [to-に] confess [if-not-なければ] forgiveness [obj-を] receive [nominalizer-こと] [subject-が] can't-do [quotative-と] [also-も] teaching-are.
A formal religious-style statement. ~については ('regarding X / as for X') scopes the discussion to a particular topic. 告白しなければ is the conditional form of ば — 'if you don't confess.' 赦しを受ける ('to receive forgiveness') uses the slightly classical 赦し (written with a different kanji from 許し to emphasize the religious/moral 'pardon/absolution' nuance). ~ことができない is the standard formal 'cannot (do) X.' The sentence-final ~とも教えています uses と (quotative) + も ('also') to add this instruction to others implied in context.