To me, "hurting your testimony" does not mean that you're not a "true" Christian. It means doing something that could be a stumbling block to a person who is not a Christian or to a newer believer. It could be letting a curse word slip out in front of the aforementioned people, or getting drunk, or condemning other people (being judgmental), etc. I know that I try not to hurt my testimony, but as a human with fleshly struggles, I have messed up a few times.