Defender of Eastern Christendom is a 
Russian decision regarding 
Ottoman Empire. Russia declares itself the defender of 
Orthodox Christians in The Ottoman Empire. It can start a chain of events, where European powers can challenge the declaration, and can potentially lead to the Crimean War.
It is visible if The Ottoman Empire is not a great power and is an absolute monarchy. It is then possible to take if relations between Russia and The Ottoman Empire is below 0 and average militancy in the Ottoman Empire is at least 3.0.
It gives Russia 0.1 prestige pr. month (adding up to 12 prestige pr. decade) for the rest of the game with the title 'Protector of Eastern Christendom' as well as give all European great powers the event The Eastern Question.
Further implications
The event, The Eastern Question gives each Great power a chance to deny Russia the title of protector. If one of them do so, Russia gets the event Protectorate Denied!. This event gives Russia two choices: Renounce the title at the loss of 20 prestige, or keep the title while giving the Great power in question a Humiliate Casus Belli against Russia. If Russia ever loses a war versus the Great power, who rejected the title, then the title and the 0.1 prestige pr. month are lost.
All in all it is a risky decision to take, and one should only take it if Russia can withstand a potential attack from any great power and their allies. It is however not given, that just because a nation rejects the title, that they will attack with the casus belli. Since it gives prestige pr. month, it is best to take as early as possible to increase the amount of prestige.
The requirements for the decision can relatively easily be provoked by attacking The Ottoman Empire.