Quote Originally Posted by boys from the dwarf
what good did she do when she was ALIVE?i knew what she did when she was dead.
(SPOILER)Aeris began the prayer at the Water Altar while she was still alive, and it was this action (simply praying for Holy) that initiated the Magic. In all likelihood, Aeris did not even need to die for the Magic to take effect. Remember that Cloud suggests to the others at the end of Disc 2 that Aeris has not yet returned to the Planet, even though we already saw during the Key of the Ancients sequence that the White Materia had begun to glow (indicating its activation).

Quote Originally Posted by scott728723745
I thought (SPOILER)Holy had the opposite effect and helped meteor destroy midgar, and the lifestream saved the planet.
That's a common misconception as to the effects of Holy. In actuality, however, (SPOILER)the Lifestream merely strengthened Holy, which by itself was insufficient in stopping Meteor. This should be obvious for several reasons. First, Sephiroth (as discussed by Bugenhagen and Cloud in the Key of the Ancients sequence) is explicitly stated to be blocking Holy. If the Magic was meant to strengthen the effects of or otherwise aid Meteor, he would never have done so. Furthermore, the White Materia is explicitly stated to do what's good for the Planet. While destroying Midgar would probably helped Gaia and its Lifestream, allowing the catastrophic impact of Meteor would have threatened the entire Planet. This is clearly outside the boundaries of the Holy magic.

During the final FMV sequences, as the party looks on, Red XIII suggests that Holy is "too late." This can be taken to mean either that Holy, in being delayed, had lost some of its potency, or that the Magic was taking place to close the the surface (Holy, perhaps, should have encountered Meteor in the upper atmosphere). Whatever the case may be, the Lifestream itself strengthened Holy in its intended function (stopping Meteor), and Gaia survived.