It is important to distinguish heresy from schism and apostasy. In schism, one separates from the Catholic Church without repudiating a defined doctrine. An example of a contemporary schism is the Society of St. Pius X—the “Lefebvrists” or followers of the late Archbishop Marcel Lefebvre—who separated from the Church in the late 1980s, but who have not denied Catholic doctrines. In apostasy, one totally repudiates the Christian faith and no longer even claims to be a Christian. (From: catholic.com/tracts/the-great-heresies.)
Does this mean that all SSPXers (who aren’t ignorant) are in schism and therefore automatically excommunicated? I’m fairly sure the answers is in the negative. But why?
The problem is with an incorrect use of terminology. The tract uses ‘schism’ in a more general concept of “falling away from the Church”, but the author forgot that ‘schism’ within an ecclesiastical context specifically means “being out of communion”. One must understand the context of this statement. The author was trying to draw a distinction between separating from the Church without rejecting doctrine, and separating from the Church while rejecting doctrine. It was not at all the writer’s intention to define whether the SSPX was in communion or not.
To ascertain this, we must understand that the Church has labelled the SSPX to be merely canonically irregular but not out of communion with the Church, and has in fact lifted the excommunications that were previously incurred by the bishops of the institute. Therefore, knowing supporters of the SSPX are not excommunicated by virtue of being in the SSPX, but the Church considers them to be committing the sin of disobedience by deliberately patronising an institute and ministers who have no canonical place in the Church.