How can sin be forgiven or redeemed
The idea of sin is an interesting spiritual concept. I'm going to approach it from the perspective not of any one religion, but in from the globally-held spiritual ideas that most hold to be true in some form or another. I will extrapolate what might seem like new concepts from the more basic ones, but they will hold rock-solid in the model based on basic fundamental concepts, however counterintuitive they might appear at first, and should withstand all scrutiny. In doing this, I not only hope to make the ideas we'll arrive at more accessible to nearly everyone with basic spiritual beliefs - rather than only members of a certain sect or religion - but also shortcut past a lot of the nonspiritual dead-ends that have cropped up in religions all over the world through the centuries.
In order to resolve the problem of sin, we must first establish what it is. Christianity, for example, expresses the idea that a Perfect and Benevolent All-Powerful Creater made a totally Perfect Creation that was without flaw. Christianity then gives us the concept of Original Sin, the idea that despite the fact that this All-Powerful, Perfect, and Benevolent Creater made a completely Perfect system, and even though there was no imperfection within it and nowhere that was outside of it, somehow imperfection was introduced into this system.
I submit that this makes no sense, and is impossible.
In a scenario like that, how would you introduce imperfection into a wholly Perfect system, outside of which nothing existed? Christianity tells us that the Fall From Grace occurred as the result of either a mistake, or an evil choice - depending upon how you look at it. But either of those, a mistake or an evil choice, would be imperfect. Thus, Christianity tells us that imperfection was introduced into a wholly Perfect Creation by... imperfection.
This makes no sense, and appears to be talking in circles. In order to have imperfection, you must first have imperfection. This does not mean anything.
At this point, many people will describe the idea that Free Will meant that there had to be a choice between Divine Will and... something else... and that Adam and Eve made the wrong decision. The argument here is that Free Will necessarily requires the existence of imperfection, and all of the suffering that this imperfection brings with it, in order to make it complete. In other words, they will say, imperfection is necessary for Free Will. Without the choice for Evil, Free Will doesn't mean anything. At this point, the person with whom you are speaking will have gone from describing a wholly Perfect Creation, beyond which nothing exists, one moment to describing how Creation, for whatever reason, must necessarily be imperfect. This does not mean anything, and makes no sense.
I can solve the dilemma quite simply. The idea that such a Creator made a wholly Perfect Creation, perfect in every way, and outside of which nothing exists, AND the idea that there would be a reason for Him to create or allow imperfection and global suffering in order to have Free Will, or to have zebras, or to have rainbows, or in fact for any reason whatsoever, is an argument for an extremely imperfect situation. And He made no imperfect situations, nor the potential for them to exist. So we must accept, temporarily setting aside what our senses have told us about the world throughout our whole lives, that Evil and suffering do not exist. We must abandon what our life-experience has told us, because it is clearly in direct opposition with the Divine facts.
We also have the option of doing just the opposite, of course. We could solve the incongruity by rejecting our notion of a Divine Creator as described, in favor of what our earthly experience and senses have taught us. But obviously one of them has to go, because the idea of that Divine Creator and the argument for imperfection, evil, and suffering our earthly experiences tell us about are mutually-exclusive.
At this point I will draw your attention to the nature of Faith. Faith is the belief in Divinity even when there seems to be no trace of it, and it seems to have left us completely. To have Faith, one must... have Faith. There's no such thing as "fair-weather Faith", because Faith means preserving a belief in Divinity precisely when there appears to be no traces of it around us. So we are discussing resolving the conflict between our earthly experience and our spiritual experience, either by choosing to keep our Faith no matter if the whole world tells us otherwise, or giving up our Faith and keeping our earthly perception of reality, no matter what we may sense in our heart of hearts.
The choice is up to you. For me, there is no choice at all. I have seen too much, and not only have Faith that is unassailable in ordinary terms, I have knowledge. To paraphrase something Terry Pratchett once wrote, for me believing in God would be like believing in the mailman.
Given that I must trash my life-experiences in the world because they conflict with what I know to be true, I have had some years to examine the nature of the earthly situation of evil, suffering and sin that we encounter daily. And I believe I have it solved, although I cannot take the credit for solving it alone. The spiritual and metaphysical ideas of mankind throughout the ages has provided bits and pieces of the puzzle. But I think I've got it.
First, let's look at the occult for a moment. Metaphysics. I have had active firsthand experiences with metaphysical and psionic phenomenon for years. I know the world to be a much stranger place than Western science would ever suggest, but this is easy for me to accept since I know Divinity to be real, and this world not. Alright, metaphysics. Many Christians, for example, have a difficult time reconciling the idea of a Creator and the idea that people have metaphysical or paranormal abilities. They deal with the apparent disparity by calling people who have those abilities evil, or influenced by evil. But consider: in his perfect state, before the supposed "Fall From Grace", Adam must have been in a more empowered state. This only makes sense, because he is said to have become enfeebled upon having made it. I submit to you that in our True and Perfect State, as the Divine made us, we are far more empowered than we are in our current condition. I further submit to you that part of our Divinely-bestowed Free Will was not the ability to choose in favor of evil - for imperfection was never created, and remains an impossibility within a Perfect Creation - but rather the ability to Choose more strongly. To Choose, and by our Choices shape Reality. What loving Creator would Create his children, but withhold the gift of Creation from them and keep it all for himself? We were meant to shape the whole world, and cultivate it like a garden, extending ourselves and our benevolence outward until it graced the whole world in our Perfect state of Grace. I submit to you that the ability for magick was Divinely-bestowed upon us, and is in fact part of our True Nature as our Creator intended. What is magick but an act of Will that affects our environment? And what is Will but our ability to Choose? In our natural state, then, we would have the ability to Choose, and by that Choice shape reality. According to Christians, what did we Choose? We Chose the fruit of that forbidden tree. But what was the fruit? It wasn't Evil. It was "the fruit of the Tree of Knowledge of Good and Evil". Which makes perfect sense.
If an omnipotent, benevolent, and Perfect loving Creator has made only Perfect Creation, and we made the Choice for "Knowledge of Good and Evil", then we Chose Knowledge of that which the Divine Created, and that which He never made, and which is impossible... Evil. We apparently Chose to shape reality itself by "Knowing" something in direct opposition to what was real and possible. Our Choice could never overwrite what our Creator had Authored, so it could not truly shape reality. But our Choice could shape our perception of reality. And I believe it did exactly that.
Okay, let me explain by taking the scenic route here for a moment. Years ago I was reading some William S. Burroughs, specifically his Book of Dreams, and he described a sort of ghostly companion in his bed with him, who was somehow both there, and not there. In case you don't know, Burroughs was an occult practitioner among other things. He called this figure a "tulpa". Being interested by the idea, I looked it up online. A "tulpa" is a word used by Tibettan Buddhist monks to describe a thought-form that has been concentrated upon so strongly that it has become physically solid. These monks go off into seclusion for a period of months and focus whole-heartedly upon visualizing... whatever it is they want to make solid. What it looks like. What it feels like. What it sounds like. The smell of it. Eventually, their Choice for it causes it to become solid after about three months. They have created items, people, even manifestations of some of their deities that would be biologically impossible by scientific standards. Their own texts say that they can create anything "up to and including the Elixer of Immortality". Which only makes sense. Whatever we can Choose, we can create for ourselves. Even if it doesn't exist.
If you've seen The Matrix, you can probably see where I'm going with this. All the "Fall from Grace" was, was a Choice for evil, which does not exist and is impossible. That Choice, however, was not without power and effect, and so it created - or appeared to create... this. The earthly world of suffering, division, loss, and imperfection you see all around you. Spacetime, which mystics from all over the globe throughout the ages have long told us is only an illusion or a dream. We apparently made a Choice in favor of the existence of Evil, and thus wove a nightmare dreamscape around ourselves which we have come to call real. Hindus call it the Maya, the Veil of Illusion. Various religions of the world call it all sorts of things. It's a pretty universal concept, that. Even the Christians, who often describe the world as real, talk of a day when it will be brushed aside by the Creator like so much useless and insubstantial seafoam.
Alright, now that we're working from my native model, I can wrap it up by addressing your question directly. "How can sin be forgiven or redeemed?" Well, sin is the Choice for Evil. More precisely, it is the Choice for the argument for the existence of Evil, which makes no sense. It's a Choice for a fantasy, a phantasm, a thing which the Creator never made and we could not have truly made either. So how do you forgive or redeem something which has no existence?
Easily. You acknowledge that it has no existence, and never did. You give your perception that it once was real - a perception that you created - up to your Creator, and allow Him in His love and mercy to extend his healing upon your vision, so you may see truly and recognize the Perfect Creation which stands for all time, never displaced, and aside from which nothing exists. In other words, you dispel shadow, which is not "real", with a recognition and acceptance of Light, which - in its spiritual sense at least - is the only thing that was ever real. Neither Space, nor Time, nor any other justification for separation, suffering, division and loss can stand against the Perfect - and Perfectly Loving - Creation that was, is and will forever be, because nothing was ever Created to stand against it. The idea is meaningless.
So if you're struggling with the concept of how to undo, forgive, absolve, or redeem sin, try looking at it as your Creator must recognize the situation. No sin ever existed, no Choice for Evil ever occurred - for that would be imperfect, and imperfection is not your Creator's Will. (You may wish to corrolate this with the Asian spiritual and political concept of the Mandate of Heaven, sometimes referred to as the Will of Heaven - a concept with more far-reaching metaphysical implications than mere political succession, with the equivalent European concept of the Divine Right of Kings, and with the Shinto mythology surrounding the Sun goddess Amaterasu - particularly that bit where the Light hid herself away in a cave after much abysmal treatment by her "evil" twin brother, and what it took to bring Light back into the world) If you've ever wondered how religions can portray the Divine Creator as loving, forgiving, and all-merciful, here is His secret: He knows that nothing ever truly happened to forgive.
The rest of life is simply a matter of making that Choice as your Creator does, the Choice not to "know" both Good and evil, but to know that only Good exists, and that only Good can. This makes it easy to overlook trespasses against us, to turn the other cheek, and so forth. And of course, integrating that Choice into our own behavior towards others, so that our behavior towards our brother is not a meaningless and insane argument for the existence of Evil, spoken by our deeds rather than our words, is just part of bringing our Choice more into alignment and accordance with our Creator's Will. Or rather, realizing that it was never truly out of alignment. We're not as bad as our earthly experience makes us out to be, and there remains little point in maintaining an argument for the contrary by Choosing what we know does not exist. At that point life becomes pretty simple, really.
I hope this has benefited you. You are welcome to reply if you would like. If you would like more information, please consult your religious materials again with this perspective in mind, and consider becoming familiar with A Course in Miracles.