Sorry. I meant to Preview Post and hit Submit Reply instead.

Then the Internet locked for a while, so ...
What BalbanesBeoulve said is correct.
The Apostolic Palace is a World Wonder unlocked by Theology.
When it is built, the state religion of the civ who built it becomes the "AP Religion" (in your case: Hinduism).
Any civs also running that religion as their state religion become Full Members. A bonus mentioned only in the Civopedia is that all AP-State Religion buildings gain +2

(for a total of +6 for Temple, Monastery & Cathedral.)
Any civs not running the AP religion as their state religion but who have at least one city with the AP religion present become Voting Members.
-- hope that clarifies things a lil.