The Greek verb diakrino means to judge between, to make a distinction, to evaluate critically — or in its reflexive/passive sense, to doubt, to be divided in one's mind. The prefix dia- suggests separation: the act of separating and weighing.
Diakrino has a rich range of uses. In 1 Corinthians 11:31, Paul urges believers to judge themselves rightly before taking the Lord's Supper. In Matthew 21:21 and James 1:6, it describes the divided mind of doubt that hinders prayer. In Acts 10:20, Peter is told not to diakrino — make distinctions — about Gentiles.
Theologically, diakrino teaches that spiritual discernment (proper judgment) is essential, but a divided heart (doubt) undermines faith. The call is to whole-hearted trust without inner division.