Choi, Chi-Won (崔致遠. 857-?) is famous scholar in the age of later Shilla-Dynasty. At the age of 13, he went to China to study. After 16 years he returned from China to Silla. He found there were unique and different religious ideas in Silla from The Three Teachings of China: Confucianism, Buddhism, Taoism. He called it Fungryu-do(風流道) in his famous inscription, Rhan-lang-bi-seo (鸞郞碑序). Fung-ryu-do is one of the ancient Korean thoughts. But there are many different opinions among the scholars. Therefore, this treatise intends to reinterpret Rhan-lang, Fung-ryu-do, Hyeon-myo-jee-do(玄妙之道), Pho-ham-sam-kyo(包含三敎), Jeop-hwa-gun-seang(接化群生) etc., mentioned in Rhan-lang-bi-seo by Choi Chi-Won. On one hand, Fung-ryu-do was the indigenous religion. On the other hand, it is something different from the Three Teachings(三敎), Confucianism and Buddhism and Taoism, which had some struggles with the indigenous religion in China. In conclusion, we can assume there is some relationship between Fung-ryu-do of Rhan-lang-bi-seo and Cheon-bu-gyeong(天符經) which was first discovered interpreted by Choi, Chi-Won. The core of Cheon-bu-gyeong is consistent with the core of the myth of Tangun, that is, harmony of Heaven, Earth, Man(天地人). Therefore we insist Fung-ryu-do is a Korean oldest religion which can be called for Faith of Three God(三神) in the myth of Tangun impling harmony of Heaven, Earth, Man.