The games aren't related at all.

Firstly, FFXII takes place in the WORLD of Ivalice. In FFT, Ivalice is a country, and in FFTA - it's a world. Secondly, the world maps do not match in any of the games - nor do any of the places (sans FFTA and FFTA2)

Further, no FF's are related unless directly specified as sequels.

That said, the Lucavi in FFT were the direct bastardization of the twelve apostles of the bible, as well as St. Ajora being a Jesus-like figure. Having a counter to the entire faith in FFT is what made the story what it was and so effective, as well as the entire struggle between the political factions.

The Lucavi serve as nothing more.