Why was there light through the door to darkness?

[Sarius]

The light that defeated Ansems heartless. I understand that they didn’t know there would be a continuation of the story but plot wise was it ever explained? It’s implied that it was from Kingdom hearts but where did it come from?

 

[Elysium]

I'd refer to the cutscene of Kairi's grandmother telling the story in KH1 (as well as the little tablet you read just before fighting Chernabog). The true light is supposed to be deep down within the darkness. "Someday a door to the innermost darkness will appear and the true light will return." (paraphrasing from memory)

 

[Xblade13]

The light that defeated Ansems heartless. I understand that they didn’t know there would be a continuation of the story but plot wise was it ever explained? It’s implied that it was from Kingdom hearts but where did it come from?

Kingdom Hearts, the real one, is within the Realm of Darkness itself, down in the deepest, blackest darkness there is. It can still shine with light, and that light is what escaped when the Door to Darkness was opened, strong enough to mostly obliterate Ansem. (Mostly, because Ansem survived in Riku until the Kingdom Hearts Encoder blew up in KH2.) Ansem was attempting to reach the True Kingdom Hearts, and failed because he misjudged it's true nature (seemingly Ansem was truly amnesiac of Xehanort's past life prior to Apprenticenort), believing Kingdom Hearts to have been the source of Darkness and not Light.

The only ways to access Kingdom Hearts appear to be:
- enter the Realm of Darkness and find it (Ansem)
- create a fake (Xemnas)
- forge the X-blade to force it to appear (Xehanort)