Sunni and Shia Islam class different things as sin; who would be a sinner if only one of them is correct?

I keep thinking about why Sunni and Shia and other muslim communities have different opinion about different issues which makes them very difficult for us to follow right path.
Let's take example. sunni and Shia views on Mut'ah are different so I don't know which one is right. We are in one religion and one religion has one law created by Allah so if something is sin by Law that means it is sin and nobody can change it. Sunni community says Mut'ah is haram and Shia community says Mut'ah is halal. What if Mut'ah is halal and we consider is haram and what if it is really haram.
Who would be responsible for a change in view? Who would be a sinner?