42 To such conduct the warning words of the Divine Teacher fittingly apply: "He who enters not by the door into the sheepfold, but climbs up another way, is a thief and a robber." The sheep indeed know the true shepherd's voice. "But a stranger they will not follow, but will flee from him, because they do not know the voice of strangers."
Source: Ad Apostolorum Principis (Vatican.va)