James 2:12-13

So in our actions and speech, we are to carry them out as one who us under the law of liberty. James is referring to his previous point about how a breaker of the law is one who breaks the law in one area. We are to remember this in our actions. More specifically, we recall the greater context about not showing favoritism. We are under this law, which is the same law in which others are under. Recalling what we are under and remembering that others are under this as well, we should show mercy to others. The point made is that those who do not show mercy are not given mercy, but only judgment. It's implied that mercy will be shown to those who are merciful. Showing favoritism would probably not be the same as showing mercy, so this is yet another reason why we should not show favoritism.