In the Catholic Worker in May 1951, Day wrote that Marx, Lenin, and Mao Tse-Tung "were animated by the love of brother and this we must believe though their ends meant the seizure of power, and the building of mighty armies, the compulsion of concentration camps, the forced labor and torture and killing of tens of thousands, even millions." She used them as examples because she insisted that the belief that "all men are brothers" required the Catholic to find the humanity in everyone without exception. She explained that she understood the jarring impact of such an assertion:  Peter Maurin was constantly restating our position, and finding authorities from all faiths, and races, all authorities. He used to embarrass us sometimes by dragging in Marshall Petain and Fr. Coughlin and citing something good they had said, even when we were combating the point of view they were representing. Just as we shock people by quoting Marx, Lenin, Mao-Tse-Tung, or Ramakrishna to restate the case for our common humanity, the brotherhood of man and the fatherhood of God.  In 1970, Day emulated Maurin when she wrote:  the two words [anarchist-pacifist] should go together, especially at this time when more and more people, even priests, are turning to violence, and are finding their heroes in Camillo Torres among the priests, and Che Guevara among laymen. The attraction is strong, because both men literally laid down their lives for their brothers. "Greater love hath no man than this."  "Let me say, at the risk of seeming ridiculous, that the true revolutionary is guided by great feelings of love." Che Guevara wrote this, and he is quoted by Chicano youth in El Grito Del Norte.

Answer this question "What does All Men Are Brothers refer to?" by extracting the answer from the text above.
the belief that "all men are brothers" required the Catholic to find the humanity in everyone without exception.