After Napolean's failure, there were many opposing opions to return to the old orders before Revolution. Romanticism also prospered and fostered the renaissance of religions. Lamennais was the most outstanding representative. There are three parts in this paper. First, I talk about Ultramontanisme, promoted by Lamennais, and the influence on the Catholic Church at that time. Second, what was Catholic Liberalism, promoted by Lamennais and why were the French Government and the Catholic Church opposed to it? Last, why did Lamennais break off relations with the Catholic Church? Did he change his ideal to establish the united Christendom? I hope to make clear his thought about the reformation, compared with that of the Catholic Church.