If truth is only what people think it is then we are in trouble on a moral level because we all have different things that influence how we view truth. I believe what you are referring to in that matter is what's known as subjective truth otherwise known as relative truth. That does happen. For example if it is summer time I walk outside with a coat on and say "it is hot" but someone else without a coat on may say "it's not hot". In that case then there is subjectivity.
I was referring to an absolute truth however. A truth that does not change depending on how it is viewed. Absolute truth can not be changed. It is the standard. It's like math or laws of nature. They don't change into new laws of nature. They have always been the same although we have discovered them and given them classifications.
So if truth is only relative of subjective then we have no standard of what is true or not or what is truly right or wrong.