The Second Council of Nicea was held 442 years after the first one, and was the last of the first seven ecumenical councils. Like all of them, it was held in what is now Turkey, and it was an attempt to keep the Catholic church unified – to punish or prevent schisms, and in general, to make sure everyone was on the same page. However, while the First Council of Nicea is one of the few pivotal events in the history of the Church that created lasting changes still well-known today, the disappointing sequel was mostly about internal administrative matters – particularly as regards the slow split between what would become the Western Catholic Church and the Eastern Orthodox Church.
While some of the decisions made at this council are still in effect today in the Catholic Church, much much more of them are still in effect in the Orthodox church. But unless you’re actually a member of the clergy of one of those two churches, it’s unlikely to be something that affects you.
