The separation of religion and state is the foundation of secularism. It ensures religious groups don't interfere in affairs of state, and the state doesn't interfere in religious affairs. By this definition, to my knowledge, there is no truly “secularistic” nation on Earth. Japan may come close as it is considered a secular democracy. Japan’s constitution, Article 20, Freedom of Religion, Secularity of State, sets out 4 points separating religion from the state.
Also, some countries may be moving toward secularism. Norway, for example, changed its constitution and declared that the Evangelical Lutheran religion would no longer be the state’s official religion.