Please read and correct the sentences! :)

Samgukyusa (三國遺事 Memorabilia of the Three Kingdoms’)
1. The [Samgukyusa] is a prototype of Korean culture and a Korean classic treasuries.

a. your first sentence should explain what Samgukyusa is.  I did not know it was a book by this sentence.

b. a prototype is used for machines, manuals, etc.  Because Samgukyusa is mainly historical accounts, it cannot be a prototype.

c.  "Korean classic" is sufficient, or you can say "Korean Treasure".  A treasury holds treasures.  So a "Korean Classic Treasury" would be a library.

  i.  The [Samgukyusa] is a series of stories written by the buddhist monk Ilyeon (一然).  It is often cited as the origin of Korean cultural and is thought of as a Korean Classic. 

2. Samgukyusa is a symbol of Korean culture’s 'ancient future.’ As the Samgukyusa was written by a Buddhist monk, it has been discussed mainly in the context of Buddhism, but it is in fact a treasury of information that includes mythology, history, language, thought, culture, philosophy, etc.

a. I have no idea what "ancient future" means.  Do you mean 'Golden Era'?  Or you could use "a halcyon period in Korean culture"

b. I moved the monk's name to the first sentence, so it is removed here.

3. In regards to Zen Master Ilyeon’s ‘forward-looking’ style of annotation and his objective style of writing, his standing as a scholar still endures, even by 21st century standards, and must be reevaluated.

a. In regards should change to "as evidenced"

b. reevaluated in this context would mean that Ilyeon is thought of as not a scholar.  It would be better to say "respected as such".

 i. As evidenced by his ‘forward-looking’ style of annotation and his objective style of writing, his standing as a scholar still endures, even by 21st century standards, and he must be respected as such.

4. His life itself is a significant source of cultural content.

a. This sentence should be before (3).  

 i. Not only his writings, but Ilyeon's life itself is a significant source of cultural content.

Great work!  If you have any questions let me know.