What does is mean for everything to be legal?

Something that often comes up, and that I don’t fully understand is Paul’s statement in I Corinthians that “all things are lawful.” He immediately follows this up saying that we shouldn’t do anything immoral. However, I still wonder what exactly does this mean in the scope of sin? I was reading I Corinthians in my morning devotions, and I had to dig deeper. Sure, I know what the Sunday school answer is. Still, I wanted more. After reading it several times it seemed to me that Paul was actually trying to impress on the Corinthians how bad sin was. Before they could break the law, and then offer a sacrifice and it would be all better. Paul first separates these ideologies by saying the law has nothing to do with it. He then impresses on them the importance of keep their bodies temples of the Holy Spirit. He seems to be trying to bring them back to the higher standard the Jesus tried so hard to get in their brains. What does this me to me? I think it reminds me how much God forgives, while at the same time trying to make me focus on WANTING to do the right thing rather than HAVING to do it.