I will define sin--several different kinds. First, you have absolute sin. Doing this is bad for oneself and/or society. As I see it, laziness, dishonesty, and envy are at the root of all absolute sin. The acts are deliberately doing what is bad for self and/or society, and refraining from doing what is good for self and/or society. Strangely enough, many people actually attribute virtue in doing this.
Biblical sin, or scriptural sin (to accomodate Muslims), means doing what your book says is bad. For Jews and Christians, a "scriptural sin" is doing something the Bible (or the part you follow) tells you is bad. Jews obey the Mosaic Law, while Christians obey Jesus' teachings as interpreted by the Gospel writers and Paul. Islams obey the Law that is found in the Quran. When you break one of these rules by doing what is forbidden or failing to do what is required by your book, you have committed a "scriptural sin". Notice that a Scriptural sin" is not always an absolute sin.
Now, there is "religious sin". This is when you break one of the rules, as interpreted by your religion. If your religion interprets an act as violating the book (Bible or Quran) it is supposedly based on, you have committed a religious sin. Usually, this is taken out of context--the people interpreting their "Holy" Book may or may not have pure motives. Breaking the Washtowel rules (celebrating holidays and birthdays, smoking, joining the military, gambling (especially gambling within reason), and saluting the flag) constitute "religious sins" since you are sinning against the religion. Same if they insist that field circus and boasting session attendance are required and you skip doing so--this, too, is breaking the rules of the religion. Note that religious sin may or may not also be Scriptural and/or absolute sin.
Forgiveness is another matter. To be forgiven from absolute sin, one must try to correct the underlying reason for it (usually it's stagnation and a society that limits fully integrated thinking, or being brainwashed by religion) and fix the mistakes you have made. Only the most extreme instances of absolute sin are unforgivable (this includes people that insist on pushing a society where everything anyone does is what they are told to).
Scriptural sins have their modes of "forgiveness". Here, one starts creating problems where none should have existed--Original Sin is a theme of both Bible and Quran. There is a "savior" in both cases, and you need to take some kind of action. Usually, acknowledging this Savior is required, and you need prayer. You also need to work on the symptoms of the sin--that is, the act or thought that you are doing "wrong". You need also to attempt to correct the "sin" you are committing. Both Bible and Quran have the pathways--studying your "Holy" Book is thus part of the requirement to get "scriptural forgiveness" for "scriptural sin".
Religious sin also has its rules. The Catholic church has confessions, and the "Three Hail Marys" is a common penance. Some religions also have penances (which are often not even in the Bible or Quran)--extra work, contributing more to the religion, or other activity might be needed or aid in attaining religious "forgiveness" (which means your sins are forgiven in the eyes of your religion). There are also religions that impose other punishments--getting marked as "bad association", being disfellowshipped, going to all the services, prayer, and showing the leaders that you have quit your "sin" and turned around are all among the requirements that various religions have for being forgiven in the eyes of the religion.
As for the witlesses, their sins are supposedly forgiven if they study the Bible (the Not Well Translated version) along with the Washtowel littera-trash, go to all the boasting sessions, and take in the knowledge. They then have to quit doing these "sins", and turn against those courses. If they relapse, they have to go to the hounders. Prayer will happen now, and there will be hounding using scriptures out of context (along with a liberal dose of Washtowel littera-trash). It is possible that you will also have to go to boasting sessions, sit silently, and have another meeting several months or years after the original "sin".