Literally: doubt – heart – dark – ghost
Alternately: Once you allow suspicion or fear into your heart, everything will start to seem menacing. Even though nothing is actually there, a doubting heart fills the darkness with monsters.
Notes: Normally I’d deal with 鬼 by leaving it as oni, but here my sources specify that the 鬼 character refers to the malevolent spirits of the dead – and the fact that we the living can work ourselves up into a state of terror over them even though they don’t really exist.
A more kotowazaesque version adds a particle and verb: 疑心暗鬼を生ず (gishin anki wo shouzu), i.e. the doubting heart “gives birth to” ghosts in the darkness.