the line between good and evil is razor-thin
Proverb meaning that the boundary between good and evil, or between heaven and hell, is extremely thin and can be crossed easily.
As the saying goes, the line between good and evil is razor-thin.
紙一重 also means a very thin difference, but is used more broadly for any subtle distinction, not specifically moral or spiritual.
A Japanese proverb combining 地獄 (hell) and 壁一重 (a single wall), literally 'hell is just one wall away', emphasizing the proximity of damnation to everyday life.