4 Yet it is easy to see how important it is that unity should exist among the minds of men, and all the more so that, amid the unfettered prevalence everywhere of error and in the war so violently and insidiously waged against the Catholic Church, it is absolutely necessary that all Christians would unite their wills and powers in resistance, for fear that separately they may be crushed by the cunning and violence of their foes.
Source: Cum Multa (Vatican.va)