Nothing real can be threatened.
Nothing unreal exist.
Herein lies the peace of God.
(A Course in Miracles)
A sin is quite easy to define. It is an action or a thought that you believe deep down inside is wrong. Essentially you stepped away from your birthright of love and peace. And the force that encouraged you to do it in the first place beats you up over it afterwards.
There is no God who punishes past "sins" for the very reason that they never existed. You literally only dreamt this stuff. But you believe it, so you need to punish yourself - consciously or subconsciously - for it. That is the law of karma.
Many spiritual masters say that you should leave the past to the past, and openly accept everything life throws at you in the holy Now. The only choice you have is to make sure that nothing else gets added to this list, and that's what a spiritual path is all about.
A sin disconnects you from God's Way. It is a thought or a feeling that is non-loving in nature. While you can do little about the thought or the feeling per se, the trick is not to give more energy to it be either by belaboring on the subject, or worse, to do something about it.
A spiritual path is not only learning about the power of discrimination, it is also a course in energy management. If you beat yourself up over the perceived wrongness of your thought or action, you only give energy to the force that got you in that place to start with. Rather, let life, with all its ups and downs, and lefts and rights, recycle that energy for you until there is nothing left but love and peace.
Relax my friend, nothing unreal ever existed. There is only love and peace.