Seventh-Day Adventist Beliefs: The Nature of Man
Man and woman were made in the image of God with individuality, the power and freedom to think and to do. Though created free beings, each is an indivisible unity of body, mind, and spirit, dependent upon God for life and breath and all else. When our first parents disobeyed God, they denied their dependence upon Him and fell from their high position under God. The image of God in them was marred and they became subject to death.
Their descendants share this fallen nature and its consequences. They are born with weaknesses and tendencies to evil. But God in Christ reconciled the world to Himself and by His Spirit restores in penitent mortals the image of their Maker. Created for the glory of God, they are called to love Him and one another, and to care for their environment.
--Fundamental Beliefs, 7
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I think that mankind has gone wrong. This is a difficult idea; it is an effort for most people to publicly admit imperfection, but to recognize that one is fundamentally flawed is even harder.
God didn't intend for humanity to fall. But it happened, because he gave people, from the very start, the free will to choose. In C.S. Lewis's Mere Christianity (which is a really good book, btw) he says this about it:
God created things which had free will. That means creatures which can go either wrong or right. Some people think they can imagine a creature which was free but had no possibility of going wrong; I cannot. If a thing is free to be good it is also free to be bad. And free will is what has made evil possible. Why, then, did God give them free will? Because free will, though it makes evil possible, is also the only thing that makes possible any love or goodness or joy worth having.
A lot of people think that because they are "good people" in the sense that they don't commit crimes like adultery or theft that they aren't "sinful" and don't need to be helped. But this is not the kind of sin we are talking about. I wrote about this at different kinds of sin, but in short, sin is separation from God. This can be from "sinful" actions, but fundamentally--in all people--there is a separation inherent in being born from someone who has been separated from God. That's what's usually called original sin, and is not from any crime on your part but is an inheritance from the Fall.
I do think that someday God will make everything right again. He will remove our inherited tendency to sin, and will reverse the effect sin has had in our lives. This is what I can look forward to as a Christian.