As if it were raining flowers

In the Southern and Northern Dynasty (420-589), in the reign of King Wu of Liang, there was a monk called Master Yun Guang who was a very accomplished preacher. Once he explained the sutra so profoundly and subtly that the God of Flowers was moved and sent divine flowers down to Earth. Soon the land was covered with flowers. 

南北朝梁武帝的时候，有一个名叫云光的法师，讲经讲得很好。传说他有一次登坛讲经，感动了天上的花神。花神把天上的花纷纷降落下来，洒遍了大地。 

This idiom was later used metaphorically to describe talking in a vivid and eloquent way (mostly in an exaggerated and imparctical manner).

“天花乱坠”这个成语后来比喻说话有声有色，非常动听（多指夸大的或不切实际的讲话）。