I think traditionalist-minded folks can get bogged down with the technicalities of the Msgr Perl letters, and there are several of them, because of nuances related to fulfilling the Sunday obligation and other particular issues.
My approach to this issue is simply this:
1. Canon law defines what "schism" is: "Heresy is the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Supreme Pontiff or of communion with the members of the Church subject to him." (CIC, 751).
2. Schism is incurred "latae sententiae", that is to say, automatically by the very act of schism, without a need for declaration from the ecclesiastical authority: "An apostate from the faith, a heretic or a schismatic incurs a latae sententiae excommunication" (CIC, 1364).
3. The SSPX, in various ways, in currently available printed books, pamphlets, tracts, articles, blogs, websites, maintains or promotes the historical teaching and statements of Msgr. Marcel Lefevbre, indicating that the SSPX cannot be in communion with "modernist Rome".
It is clear that with all those who leave us or who have left us for Sedevacantism or with those who want to be subject to the current hierarchy of the Church while hoping to keep the Tradition, we can no longer have relations with them. It is not possible. We say that one cannot be subject to ecclesiastical authority and keep Tradition. They claim the opposite. This is to deceive the faithful. (…) we want to be absolutely free from compromise both with regard to Sedevacantists and with regard to those who want to be subject to ecclesiastical authority. -Archbishop Marcel Lefebvre, Conference in Flavigny, December 1988, Fideliter n° 68, p. 16.For more evidence of the teaching of Lefevbre: Look here, here, here, here, and here
4. Therefore, the SSPX, at least in particular cases, that can be easily verifiable by asking their priestly membership, or followers, 1) if they submit to the Supreme Pontiff or 2) accept communion with the members in union with the Supreme Pontiff, or 3) affirm or reject the various schismatical teachings of Msgr. Lefebvre on the SSPX relationship with the Roman authorities can be shown to meet the definition of "schism" (CIC 751) and incur such a penalty automatically without ecclesiastical declaration (CIC 1364).
5. Msgr. Perl writes that the Pont. Commission Ecclesia Dei discourages attendance at the SSPX, and that while attendance itself is not a "formal schism", one might adopt the "schismatic mentality", which would then incur the penalty of schism.
In summary:
- Canon law defines schism (Can. 751).
- The SSPX priests and their followers can be shown to hold a "schismatic mentality",
- This meets the criteria for schism.
- It is incurred automatically (Can. 1364).
- Therefore, it is not advisable to attend the SSPX chapels or socialize among their communities.
- "At this time, they (SSPX) are not part of the one Roman Catholic Church throughout the world" - Cardinal Raymond Burke
- Schismatics, the SSPX, and Sedes w/ John Salza (Pints with Aquinas)
- SSPX Masses and the Sunday Obligation by John Salza
- The ERRORS of Archbishop Lefebvre and the SSPX w/ John Salza (Robert Sungenis LIVE)
- Is the SSPX REALLY Doctrinally Sound? w/ John Salza
- John Salza Responds to Peter Kwasniewski on the SSPX
- When the SSPX physically attacked the FSSP chapel in Guadalajara over "false ecumenism"
- From Sede To Catholic (w/ Dan Kavic) - SSPX a Cult?
- True or False Pope: SSPX Page (John Salza and Robert Siscoe)