Did Gratian view abortion as a mortal sin?
It is important to note that there is a difference between canon law and moral law. Not every violation of the moral law is a violation of canon law. Canon law also is a true legal system with it its own inner logic.
A particular sin may be a grave violation of the moral law, but unless it is defined as a crime in the canon law, there is no ecclesiastical penalty. Canon law, like all legal systems, uses terms in a precise manner, and therefore in order for an action to be considered to be a particular crime, the action must meet the specific meaning of the terms used in canon law.
This is important when we discuss the relevant quote from Gratian on this matter. He wrote in the Decretum:
He is not a murderer who brings about abortion before the soul is in the body (Concordia discordantium canonum, Decretum, Ad. c8, c. XXXII, q. 2).
In this statement Gratian is not referring to the moral law but to the canon law. This statement was based on that era’s understanding of biology in which it was generally thought that the fetus did not become human until later in its development. It was commonly thought that until the fetus was “animated” no soul was present and thus fetal development was divided into “pre-formed” and “fully formed.” Therefore, based on how the legal system of canon law defined murder and how that era understood biological development, Gratian stated that abortion of a “pre-formed” fetus was not the legal/canonical equivalent of murder.
However, abortion had been unanimously rejected as gravely immoral since the beginning of Christianity This teaching goes all the way back to The Didache, which was written in the first century. The legal debate about what technically constitutes murder also existed long before Gratian complied his Decretum and was deemed irrelevant to the moral discussion by theologians:
A woman who deliberately destroys a fetus is answerable for murder. And any fine distinction as to its being completely formed or unformed is not admissible among us (St. Basil the Great, Epistle 138, c. A.D. 375).
Thus the distinction between an abortion of an “animated” or “fully formed” fetus and an abortion of a “pre-formed” fetus was merely a technical one relating to what ecclesiastical penalties were attached to the action. It was not a discussion of the morality of the action.
Modern biology tells us quite clearly that from the moment of conception a human life is present. Thus the legal and canonical wrangling of the Middle Ages is obsolete to present-day discussion.