The brightest light of the American Church in the 20th century is among the many clerics who have called out the SSPX for their schismatic behavior.
Ten years before the schismatic consecrations by Abp. Marcel Lefebvre, His Excellency Fulton J. Sheen foresaw the road the SSPX was travelling down. At the time (1978), the SSPX had already been canonically suppressed, and Lefebvre had been ordaining priests against the express will of the Holy Father, earning him a suspension a divinis.
After receiving a letter from a concerned woman regarding the SSPX, Sheen penned a response warning of the grave consequences of dabbling with the "schismatical sect."
The authenticity of this letter is undisputed, even by the SSPX itself. The letter was printed in The Angelus, the SSPX's publishing arm, shortly after it was written, along with a response rebuking Sheen.
September 21, 1978
I thank you for your kind letter and I admire you as the mother of eight small children. I am sure you are busy, but happy.
If you have any influence on your friend I would beg you to influence her to leave the so-called Society of Saint Pius X. This group has no ecclesiastical approval, and indeed, it can lead her and possibly her family into schism and even heresy.
The Vatican Council approved the updating of the Liturgy and amongst the changes were those recommended for the Mass. The changes made by Pope Paul VI were not doctrinal changes, they merely changed from Latin to the vernacular. There have been many changes in the Mass down through the centuries.
The Lord never said Mass in Latin; He used the language of the time. Moreover, the change in translation does not alter the meaning of the text. I am always looking for translations that make the Scriptures more understandable and clear.
Since I never write to anyone unless they have written to me I shall not write to Mrs. Richardon. I beg of you to tell her that she should withdraw from that schismatical sect as soon as possible, or suffer the consequence of possibly finding herself outside the Church.
God love you!
†Fulton J. Sheen