17 In truth, the danger of formal cooperation, or of any approval whatever of the present law, is removed, as far as is necessary, by the protests energetically expressed by this Apostolic See, by the whole Episcopate and the people of Mexico. To these are added the precautions of the priest himself, who, although already appointed to the sacred ministry by his own Bishop, is obliged to ask the Government for the possibility of holding divine service; and, far from approving the law that unjustly imposes such a request, submits to it materially, as the saying is, and only in order to remove an obstacle to the exercise of the sacred ministry: an obstacle that would lead, as We have said, to a total cessation of worship, and hence to exceedingly great harm to innumerable souls. In much the same manner the faithful and the sacred ministers of the early Church, as history relates, sought permission, by means of gifts even, to visit and comfort the martyrs detained in prison and to administer the Sacraments to them; yet surely no one could have thought that by so doing they in some way approved or justified the conduct of the persecutors.
Source: Acerba Animi (Vatican.va)