Just to add one more point: normally the Church (and therefore the faithful) require a MIRACLE to prove that a message is from heaven. No miracle, no locution.
Even then, "miracles" can be simulated by the devil, and a "miracle" by itself does not guarantee any authenticity. It's only the Church's judgment that can result in moral certainty, of at least "safety," of heeding the words of an alleged locution. That's my issue with Bishop Williamson's use of alleged "Eucharistic miracles" to "prove" the potential validity of the NOM. It would be childsplay for a demon to swap out a host with some human heart muscle (and then to keep it fresh) ... assuming that's even what those phenomena were in Poland, for instance, rather than some red mold. #1 criterion of the Church in evaluating any private revelation or miracle is its theological orthodoxy. Because the NOM Eucharistic Miracles give the impresion that the sacrilegeous bastardized Prot Liturgy can be not only valid but even pleasing to God (to confirm it with miracles), that would instantly rule them out on the basis of heterodoxy. We don't derive our theology from private revelation, but judge private revelation on the basis of Catholic theology.