Columbia subject headings:
Catholic Church--Apologetic works.
Catholic Church--Doctrines--History.
Catholic Church--Relations--Protestant churches.
Communism and Christianity--Catholic Church.
Protestant churches--Relations--Catholic Church.
Theology, Doctrinal--History--20th century.
Harvard subject headings:
Catholic Church--Apologetic works.
Catholic Church--Doctrines--History.
Catholic Church--Relations--Protestant churches.
Communism and Christianity--Catholic Church.
Protestant churches--Relations--Catholic Church
Theology, Doctrinal--History--20th century.
Princeton subject headings: