But he who doubts is condemned if he eats, because he does not eat from faith; for whatever is not from faith is sin.
Romans 14:23 NKJV

Sin doesn’t necessarily mean doing bad things. The definition of sin is very simple—everything not from faith is sin.

So sin takes place when human’s mind not being in line with faith.

And I believe one of the first sin in the history is man’s not believing in God’s unconditional love.

There is nothing that can separate us from the love of God (Romans 5:6-8, Romans 8:38-39). And Adam and Eve didn’t have the faith to believe it and so they hid from God because of their nakedness.

***There were also many other things happened right after man’s eating the fruit of the tree of the knowledge of good and evil, which will be discussed sooner or later. 🙂 ***