What makes something a sin?

Since Christians believe that the Bible is God’s Word, that means’ there’s no higher authority. Stated positively, it means that we do as the Bible commands. But it also means that we don’t add commands to the Bible that’s aren’t there. To say it another way, nothing is a sin that the Bible doesn’t classify as sin, either by clear teaching or by principle. I grew up in a tradition that tended to do this. For example, we thought it was sinful to use playing cards because some people use cards for gambling. The proper way to think about our behavior is that the Bible is sufficient in what it teaches so we don’t add to or take away from any of it’s commands.