"From this point error soon goes on to its natural end, which is to assert supremacy."

"Somewhere on earth, if the gates of hell have not prevailed against the Church, there is a Communion whose fellowship involves no departure from a solitary article of Christian faith—and no man should be willing to be united with any other Communion.  The man who is sure there is no such Communion is bound to put forth the effort to originate it.  He who knows of no Creed which is true to the Rule of Faith, in all its articles, should at once prepare one that is.  Every Christian is bound either to find a Church on Earth, pure in its whole faith, or to make one.  On the other hand, he who says that the Church is wrong, confesses in that very assertion, that if the Church be right, he is an errorist; and that in asking to share her communion while he yet denies her doctrine, he asks her to adopt the principle that error is to be admitted to her bosom, for as an errorist and only as an errorist can she admit him.

     "But the practical result of this principle is one on which there is no need of speculating; it works in one unvarying way.  When error is admitted into the Church, it will be found that the stages of its progress are always three.  It begins by asking toleration. Its friends say to the majority:  You need not be afraid of us; we are few, and weak; only let us alone; we shall not disturb the faith of others. The Church has her standards of doctrine; of course we shall never interfere with them; we only ask for ourselves to be spared interference with our private opinions.  Indulged in this for a time, error goes on to assert equal rights. Truth and error are two balancing forces. The Church shall do nothing which looks like deciding between them; that would be partiality.  It is bigotry to assert any superior right for the truth.  We are to agree to differ, and any favoring of the truth, because it is truth, is partisanship.  What the friends of truth and error hold in common is fundamental.  Anything on which they differ is ipso facto non-essential.  Anybody who makes account of such a thing is a disturber of the peace of the church.  Truth and error are two co-ordinate powers, and the great secret of church-statesmanship is to preserve the balance between them.  From this point error soon goes on to its natural end, which is to assert supremacy.  Truth started with tolerating; it comes to be merely tolerated, and that only for a time.  Error claims a preference for its judgments on all disputed points.  It puts men into positions, not as at first in spite of their departure from the Church’s faith, but in consequence of it.  Their recommendation is that they repudiate that faith, and position is given them to teach others to repudiate it, and to make them skilful in combating it."

     Charles Porterfield Krauth, The conservative reformation and Its theology:  as represented in the Augsburg Confession, and in the history and literature of the Evangelical Lutheran Church (Philadelphia:  J. B. Lippincott & Co., 1871), 195–96.  I was put onto this by Robert A. J. Gagnon.

     Krauth was a conservative Lutheran, but the Catholic Church has long insisted upon this as well, as even Dignitatis humanae 1 (of 7 December 1965), the Declaration on religious freedom, appears to maintain.  I quote that as translated in Tanner, vol. 2, 1002:

all people are bound to seek for the truth, and when they have found it to embrace and keep it.

Homines vero cuncti tenetur veritatem praesertim in iis quae Deum eiusque ecclesiam respiciunt, quaerere eamque cognitam amplecti ac servare. 

But since people's demand for religious liberty in carrying out their duty to worship God concerns freedom from compulsion in civil society, it leaves intact the traditional catholic teaching on the moral obligation of individuals and societies towards the true religion and the one church of Christ. 

Porro, quum libertas religiosa, quam homines in exsequendo officio Deum colendi exigunt, immunitatem a coërcitione in societate civili respiciat, integram relinquit traditionalem doctrinam catholicam de morali hominum ac societatum officio erga veram religionem et unicam Christi ecclesiam.

(This whole question is, of course, very widely discussed.  For just one view, see, for example, Avery Cardinal Dulles, "Religious freedom:  innovation and development," First things no. 118 (December 2001):  35-39.)