From Romano Amerio, Iota Unum, Paragraph 153:
The present idea of dialogue obscures the way of useful ignorance that is appropriate for minds that are incapable of adopting the way of examination, and that adhere firmly to their fundamental assent and do not devote much attention to opposing views, to find out where their error lies. Being afraid of ideas opposed to what they know is certainly true, they keep themselves in ignorance to preserve the truths they already possess, and shut out false ideas and also any true ones that happen to be mixed in with them, without separating the one from the other.
This way of useful ignorance is legitimate in Catholicism, is based on the theoretical principle explained earlier [Having established even by one convincing consideration that religion is true, the latter is to be held on to even if particular difficulties remain unresolved.], and is moreover the condition in which the great majority of all religious believers find themselves.11
11 The theory of useful ignorance is developed by Manzoni in his Morale Cattolica, ed. cit., Vol. II, pp.422–3 and Vol. III, p.131.