The core of what Vatican II taught about religious liberty is contained in the first two paragraphs of this document:
Religious freedom, in turn, which men demand as necessary to fulfill their duty to worship God, has to do with immunity from coercion in civil society. Therefore it leaves untouched traditional Catholic doctrine on the moral duty of men and societies toward the true religion and toward the one Church of Christ. (1)Religious freedom, then, is not about giving rights to error. It does, however, entail toleration of error at times. Fr. William Most (who has two essays on this subject here and here) makes this point (in the first linked document):
This Vatican Council declares that the human person has a right to religious freedom. This freedom means that all men are to be immune from coercion on the part of individuals or of social groups and of any human power, in such wise that no one is to be forced to act in a manner contrary to his own beliefs, whether privately or publicly, whether alone or in association with others, within due limits. (2)
Pius XII taught in Ci riesce [that] the common good of the universal Church requires that error be permitted. In fact, in determined circumstances, God does not even give the state a right to suppress erroneous things, namely, when the common good of Church and state call for tolerance. ... "Can it be that in determined circumstances, He [God] does not give to man any mandate, or impose a duty, finally, that He gives no right to impede and to repress that which is erroneous or false? ... Christ in the parable of the cockle gave the following admonition: Let it be that the cockle grow in the field of the world along with the good seed, for the sake of the harvest." (Pius XII, Ci riesce, Dec. 6, 1953. AAS 45; Cf. Mt. 13:24-30.)That's all for now. Back to work!
COMMENT: We notice he said that "in determined circumstances" God does not even give a right to repress. What are these circumstances? A bit farther on he added: "He [the Catholic statesman] in his decision will let himself be guided by the harmful consequences which arise with tolerance, compared with those that will be found in the international community by way of the acceptance of tolerance. ... in such individual cases, the attitude of the Church is determined by the preservation and in consideration of the common good, the common good of the Church, and of the State in individual states on the one hand, and on the other hand, the common good of the universal Church."