The word "miracle" suggests, through common usage, a positive thing. Has it always? Or, like "awe", did it used to simply mean "momentous"?

In English, the word is heavily associated with what the Christian god does. However, the devil also is said to perform "miracles":

Þe tauerne ys þe scole of þe dyeule..þer huer he makeþ his miracles, zuiche ase behoueþ to þe dyeule. — Ayenbite of Inwyt, 1340

Translation: "The tavern is the school of the devil... where he does his miracles, such as it behoves the devil (to do)."

In the next sentence, the devil's miracles are contrasted with god's.

The antichrist also is said to perform miracles. The term "false miracle" is sometimes used instead, such as in some translations of the Bible.

There are also plenty of examples of (usually good) miracles being associated with non-Christian figures such as pagan gods and Mohammed ("Mahomet").

See the Middle English Dictionary for some more examples.