There is no such thing as 'sin', the only 'sin' for us is ignorance and laziness emerging from ignorance, which we punish ourselves with.
After death the average person's soul remains on the astral. It remains there on average about 15-40 years before it is forcibly reincarnated into a similar genetic line of one's family. Often the person is basically 'stuck' and many times does not even know they are dead. When they are reincarnated they lose all knowledge of the previous life and the death period.
Some with more powerful souls (as HP Cobra said, 'level 2 souls') with exceptional abilities in life who wish to watch over others or those with exceptional abilities who died in extreme rage for whatever reason can remain longer than this. These souls often incarnate with more knowledge of their past lives, or at least, a stronger more balanced inner core.
Those heavily tied into the enemy (i.e. throughout lifetimes, through ridiculous acts of self annhilation for the enemy or extreme treachery on behalf of the enemy, not the average xian) go into the enemy 'light' and their energy is absorbed.
Extremely weak souls dissipate.
SS souls or any soul that is asked by an SS to incarnate (a request to a Demon or magickal work) are reincarnated immediately or taken to Duat.
There are many possibilities beyond that but this is the overview of the average case.
The purpose of Magnum Opus and reuniting the soul is to stop this cycle.