This paper examines the friendship between Chŏng Yakyong and Yi Kigyŏng and its evolution. Their friendship rested on many commonalities: they both passed the doctorate competition together successfully in 1789, they belonged to the same political faction, the Namin, they were both officials at the court of King Chŏngjo in Chosŏn Korea, and both had important roles and contributions in this period of great socio-political, religious, cultural, and intellectual transformations. Being interested in different areas of knowedge from the West that had reached Chosŏn Korea, Chŏng Yakyong, or Tasan, adopted the Christian faith in 1784 before publicly renouncing it in 1797 due to criticism and slander. His friend Yi Kigyŏng, however, did not believe his apostasy and became his enemy. By studying the official and personal documents available, we attempt to reconstruct the vicissitudes of their friendship that resulted in Tasan’s long and painful banishment.