But to say an act is objectively evil is not to say anything about the seriousness or gravity of the act. An act can be objectively evil without being grave, or grave without being objectively evil. Allow me to demonstrate.
Stealing is objectively evil. It is never permissible to take property that does not belong to you and that you have no right to. It would be an objectively evil act to steal
Going to war is not objectively evil. If the condition of just war theory are met, the war is an act of justice and is not evil to enter into. But if a country enters into an unjust war, this would be gravely evil. This is because wars, even small ones, cause great suffering, death and destruction. That is of course why the rulers of a nation must be especially careful in examining whether or not the criteria for a just war have been met before committing themselves to such a course of action.
I think the confusion that sometimes arises over the distinction between whether or not an act is objectively evil and whether or not an act is gravely evil arises because of the issue of abortion. Abortion is both objectively evil--it is an act that can never legitimately be done--and gravely evil--because it is the killing of an innocent human being. People understand both of these facts, but since they are so used to referring to abortion as objectively evil, they begin to associate the grave matter of a sin with the formal category of objectively evil actions.
It is important to remember the distinction between the formal nature of the act and the matter of the act. Abortion is an act that is both formally evil and materially grave, but acts can also be neither formally evil nor materially grave, formally evil but not materially grave, and not formally evil but materially grave. Drawing proper distinctions in this fashion does nothing to lesson the evils of abortion, but it does allow us to properly judge other actions based upon both formal and material considerations.