不是苦心人不知

“If you do not exert yourself, you cannot understand.”
Dahui Zanggao (1089-11763, 慧宗杲)

不 not
是 this/is
苦心 literally: “suffer mind”, to make an effort, to “take pains” (Sino-Korean: 고심 goshim)
人
知 understand

“Collected Works of Korean Buddhism, Volume 7-1, Gongan Collections 1”, pp. 573-574