First and foremost, it is the duty of all Catholics worthy of the name and wishful to be known as most loving children of the Church, to reject without swerving whatever is inconsistent with so fair a title; to make use of popular institutions, so far as can honestly be done, for the advancement of truth and righteousness; to strive that liberty of action shall not transgress the bounds marked out by nature and the law of God; to endeavour to bring back all civil society to the pattern and form of Christianity which We have described. It is barely possible to lay down any fixed method by which such purposes are to be attained, because the means adopted must suit places and times widely differing from one another. Nevertheless, above all things, unity of aim must be preserved, and similarity must be sought after in all plans of action. Both these objects will be carried into effect without fail if all will follow the guidance of the apostolic see as their rule of life and obey the bishops whom the Holy Spirit has placed to rule the Church of God. The defense of Catholicism, indeed, necessarily demands that in the profession of doctrines taught by the Church all shall be of one mind and all steadfast in believing; and care must be taken never to connive, in any way, at false opinions, never to withstand them less strenuously than truth allows. In mere matters of opinion it is permissible to discuss things with moderation, with a desire of searching into the truth, without unjust suspicion or angry recriminations.
–Leo XIII, Immortale Dei 46