So in book 5, we learn more about Ivan, because he decides to explain to Aloysha his method of thinking. Their conversation was about God and Christianity. Ivan believes that the paradise and harmony people seek is priced too highly (on the suffering of mankind for their sins). He also tells Aloysha the story of the Grand Inquisitor, which basically explains that mankind need to see miracles with their own eyes in order to believe in something, and that man is only happy when they have someone to warship. I think the story was also talking about the corruptness of the church-- the church built on the mystery of Christ in order to gain power over man, and then changed the "rules" in order to allow for the redemption of sin,ect. so that even if mankind seemed happy during life, after they died, they were doomed. According to Ivan, Christ doesn't want to (or feel he has to) prove that He is indeed the son of God.
After his long conversation with Aloysha, Ivan saw Smerykdov, who told Ivan to go to Cheramyshna and that if he did, nobody would be around to protect Fyodor from Dmitry. Smerykdov was hinting at the fact that if Ivan wanted Fyodor dead, he should leave town.