The root of the current religious indifferentism, or the theory of the alleged divinely-willed character of other religions, is to be found in some ambiguous phrases of the documents of the Second Vatican Council—especially in its Declaration on the Church’s Relation to Non-Christian Religions, Nostra Aetate. Describing Buddhism, for example, the Council states uncritically that “it teaches a way by which men, in a devout and confident spirit, may be able either to acquire the state of perfect liberation, or to attain, by their own efforts or through higher help, supreme illumination.” Pope Benedict XVI himself pointed out the weakness of this conciliar document, saying: “It speaks of religion solely in a positive way, and it disregards the sick and distorted forms of religion.” Also ambiguous is the already-mentioned statement from Lumen Gentium n. 16, which says that we Catholics and the Muslims together adore the one God (“nobiscum Deum adorant”).
The other root, which we also mentioned, is to be found in the affirmation of Dignitatis Humanae that the choice even of a false religion—including the worship of the “supreme divinity” (Dignitatis Humanae, n. 4) — is a natural right of the human person (“in ipsa eius natura”: Dignitatis Humanae, n. 2). However, the natural right of the free will of the human person consists only in the choice of what is morally and intellectual good, i.e., the choice of virtue and of the one true religion, not just of the “supreme divinity.” The abuse itself of free will, however, in choosing evil (sin) and error (false religion), is never positively willed by God.
— from his new book with Diane Montagna, Christus Vincit