Hanja became prominent in use by the elite class between the 3rd and 4th centuries by the Three Kingdoms. The use came from Chinese that migrated into Korea. With them they brought the writing system Hanja. Thus the hanja being used came from the characters already being used by the Chinese at the time. Since Hanja was primarily used by the elite and scholars, it was hard for others to learn, thus much character development was limited. Scholars in the 4th century used this to study and write Confucian classics, which were written with these traditional characters as opposed to the not yet invented simplified characters. Character formation is also coined to the Idu form which was a Buddhist writing system for Chinese characters. This practice however was limited due to the opinion of Buddhism whether it was favorable at the time or not.