There is nothing to worry about as a Catholic. Go ahead and read it. It’s one of the very best novels ever written. I can sum up for you, in a few lines, what the “Grand Inquisitor” business is about, without giving anything away. No spoilers here.
Two half-brothers, Ivan and Alexei Karamazov, who have scarcely met since they were kids, are having dinner together in a restaurant in the small town where their father lives. Ivan, the older of the two, who is now a journalist living in Moscow, dominates the conversation. Young Alexei, a novice in the local Orthodox monastery, asks a question from time to time, but it’s a very one-sided conversation, in the course of which Ivan recites his long “poem” titled The Grand Inquisitor. The poem has two purposes in the novel, neither of which is particularly relevant to the storyline. On the one hand, Dostoevsky uses it to convey his own view of the differences between the Orthodox Church (which he likes) and the Catholic Church (which he doesn’t); on the other, Ivan is trying to impress his kid brother with his own breadth of mind as a progressive big-city liberal, holding views about conventional morality which will be questioned later on in the novel, when both Ivan and Alexei get mixed up in a murder mystery.