Pope Emeritus Benedict XVI with Archbishop Emeritus Rowan Williams
Not so fast! Many Ultratraditionalists would have us think that post-conciliar popes, bishops, priests, deacons and laypersons using the term “separated brethren” is heresy. This should silence those Ultratraditionalists.
“O sorrowful Mother! intercede for our separated brethren, that with us in the one true fold they may be united to the supreme Shepherd, the Vicar of thy Son.”
~Leo XIII (Amantissima Voluntatis)
Saint Pius X, famous among Ultratraditionalists, called Protestants “dissident brothers” (cf. Ex Quo). Désiré-Joseph Cardinal Mercier described us Catholics as “the envy of our separated brethren” . Pius XI described the Eastern Orthodox as “our Schismatic brethren” . Venerable Pius XII prayed “zealously for the happy return of our separated brethren in the East to Us” . Before the Second Vatican Council, Blessed John XXIII wrote to “Our brethren and sons who are separated from the Chair of Blessed Peter” .
It would be understandable for the Church to use such a term for Protestants, Eastern Orthodox, Oriental Orthodox and Assyrians, all who though not Catholic are Christians like us since they too have a valid Baptism. The Council of Trent declared, “If anyone shall say that the baptism, which is also given by heretics in the name of the Father and of the Son and of the Holy Spirit, with the intention of doing what the Church does, is not true baptism: let him be anathema” (Session VII, Canon 4; Denzinger 860).
 Mercier, Désiré-Joseph Cardinal. The Condemnation of Modernism. <http://www.cfnews.org/Mercier-Modernism.htm>.
 Pius XI. Ecclesiam Dei. <http://www.papalencyclicals.net/Pius11/P11ECCLE.HTM>.
 Venerable Pius XII. Orientalis Ecclesiae. <http://www.papalencyclicals.net/Pius12/P12ORIEN.HTM>.
 Blessed John XXIII. Ad Petri Cathedram. <http://www.papalencyclicals.net/Pius12/P12ORIEN.HTM>.