1. What does it mean?
성지 literally means a sacred place. Internet users apply it jokingly to a memorable thread, especially an old prediction that appears to have come true, and may leave 성지순례 comments when revisiting it.
Also written or said as seongji
2. The nuance
Outside a thread, it can also describe a must-visit location for fans or a hobby. Calling a prediction 성지 does not prove that its author has special foresight.
3. Examples
“예전에 쓴 예상 글이 맞아서 댓글들이 성지순례 중이야.”
An old prediction turned out right, and people are revisiting the thread.
“이 장면 촬영지가 팬들 사이에서 성지가 됐대.”
The filming location for this scene has become a must-visit spot for fans.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.