The environment of the miraculous requires the intervention of the supernatural. Often, the effects are indistinguishable from that of etheric manipulation, however the mechanics and beliefs are quite different.
The great challenge in creating a rules system for these events is in the variability of player and societal religious experience. As twentieth century [English-speaking] players we (generally) have had little direct exposure to religious practice, except perhaps, institutional Christianity in one of it's extremely varied forms. Mostly, we've been encouraged to discount such beliefs as "superstitions" and "myths". The challenge therefore in role-playing our characters is two-fold.
The fiction and mythology from which the fantasy role playing game unfolds includes many unfamiliar forms of religious expression. The "bad guys" are often followers of strange cults including, but not limited to, institutional idolatry, varied implementations of Satanism and maniacal megalomania. Our heroes practice thinly veiled Greco-Roman Polytheism, humanistically re-interpreted Buddhism from our martial-arts connection, and even a few Christians in the Arthurian tradition.
The most common unifying institution of medieval Europe is the Catholic Church. This common system of belief is used in BtM to simplify the rubric of the supernatural. As a monotheistic religion the number of permutations is theoretically one and thus the rules of interaction can be modeled in a straightforward manner. Two significant systems of belief competed with Christianity in the period, Islam and animism.
Islam is also mono-theistic and therefor may use the same system put forward for Christian miracles without huge conflicts. The prayer and rituals will clearly be different, but the mechanics are the same.
Animism was the scourge of "organized" religions. The practices and beliefs of animism often continued to be held even after "conversion" to organized religion. Much of what we know as "superstition" has its root in the animistic tradition (sometimes referred to as the "Old Ways" or some such). The basic tenet of animism is that a place, thing or animal has a spiritual (or etheric) existence with the ability of thought, will and deed. Thus the spirit of Sky can be called upon to bring rain, or the spirit of a particular cave to protect something hidden there. These spirits are traditionally cantankerous, playful and egoist. Something must often be given or promised to them in return for their assistance, such as the offering of a virgin to the Volcano in return for his continued quiescence.