John Henry Newman (1801-1890) is one of the most important theologians in the 19th Century. He is a productive writer in the areas of theology, philosophy, literature and education. Despite of being regarded as "occasional" writer, some of Newman's ideas are rather consistent and even systematic, such as his argument on conscience as a proof of the existence of God. This article explores Newman's idea on conscience and how it functions as a proof of the existence of God.