I believe we can comprehend the basic difference between them, if we take Medical Science as an example. There, we have doctors who belong to the Medical Association; if a doctor is not a member of the Medical Association, he cannot exercise the medical profession. For a doctor to be legal, he must not only be a graduate of a recognized Medical School, but also a member of the Medical Association. The same prerequisites apply to lawyers. These sciences undergo continuous monitoring, because, if one were to deviate from the proper exercising of his profession, he is tried by the pertinent authority of the professional Association he belongs to, and is expelled from the Body of that profession.
The same however occurs in the Church. The respective procedure within the Body of the Church – that is, the excision of a member – is called “excommunication”; if it relates to a member of an ecclesiastic rank, then it is called “unfrocking/deposition”. It is how heretics are excommunicated from the Body of the church. Just as within the Medical realm a “quack” (fake doctor) cannot possibly be permitted to heal, thus also in the Church it is not possible for a heretic to heal the souls of people; because, being a heretic, he does not know how – nor is he able – to heal. Therefore, in the same manner that a union between a “quack” Association and the Medical Association is never possible, it is likewise never possible for a union to take place between the Orthodox and heretics. An actual doctor is not one who merely reads many medical books, but rather, the one who has graduated from the Medical School of a University, and at the same has apprenticed over a significant period of time near an experienced professor of proven ability to heal the sick.
—Protopresbyter John S. Romanides, University Professor – from the book “Patristic Theology”