Is evil is necessary for good to exist?

Often people -- particularly religious people -- say that evil exists so that we know what good is. I think this is wrong, because a thing is what it is regardless of whether it can be contrasted with an opposite. Also, this idea seems to imply a tautology that evil is non-good and good is non-evil. I think this just an "out" that people give to God. So does the existence of good require the existence of evil?