What is sin? It is going against God’s commandments. Simple.
And what are these commandments? The famous Ten, of course.
Of course, these commandments are like pigeon-hole drawers or locker boxes. Each is representative of a set of related sins, says one spiritual writer. For example, in the first commandment (which is to love God above all things), the sins of despair, belief in superstitions, practice of black magic, etc. are grouped together. Thus the sins of adultery, masturbation, and pornography are lumped together in the sixth commandment. And so on.
So if we violate any of those “pigeon holes”, we commit sin.
And definitely, God did not make those rules just because. Unlike some sadistic high school teachers (may they be converted, those lovely pitiful beings!), God gave Moses those commandments as any manufacturer would to a user of its product. God gave us life on earth; he also gave us a manual on how to live it.
Believe it or not, the Ten Commandments (or the Decalogue, as theology buffs would say) are our guidelines to live a happy life here on earth, a life which leads to the perfectly happy and everlasting Life in heaven. They’re God’s version of Lifehack’s top-tens.
And that, my friends, is the summary of this thing called “sin”, which is the only real evil. I end abruptly.