He rejected, in his 1928 encyclical, "Mortalium animos", the idea that Christian unity could be attained by establishing a broad federation of many bodies holding conflicting doctrines; rather, the Catholic Church was the true Church of Christ. "The union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it." The pronouncement also prohibited Catholics from joining groups that encouraged interfaith discussion without distinction.