NOBODY can explain the time loop.

(SPOILER)Okay, so you beat Garland at the beginning of the game. He then gets sent back in time where he sends the 4 Chaos guardians (fiends to you old school heads) into the present. The 4 Chaos guardians then steal the power of the crystals and send it back in time to Garland who feeds off of it to become stronger and become Chaos. Some people think there's more to it than that, but it there is, it's not very well explained.