Story
In the manga series the Archangel Selakiel, who became the god of Celestia, created human beings. But just as Shadow is born from Light, Evil is born from Good. Thus, at the same time, a large number of demons were born into this world.

And then, the 13 Knights led by Vaul the Prophet appeared...at the end of a battle which cost the lives of many of their own. The 13 Knights sealed away the demons and saved Celestia, or so it should have been. Now a thousand years later, the demons have reappeared...and this time, the Messiah has been called to save Celestia.

Taken from the Holy Scriptures of the Old Church of Selakiel:

"When the Earth begins to perish, the Messiah will descend from the Heaven on wings of light. Clad in darkness, he will be so little yet he will be so immense surely he shall bring salvation to everyone. The four saints shall appear to offer him their assistance. One will give the Messiah courage, one will give the Messiah light, one will give him wings, and one will shield from his enemies."

The Messiah that has been foretold in these scriptures is none other than Shinya Yumiki. (wiki)