From kleptō (G2813, to steal). Kleptēs means a thief, one who steals by stealth rather than by open violence (contrast with lēstēs, a robber or bandit). Used both literally and as a metaphor for the devil, false teachers, and the unexpected return of Christ.
The word kleptēs operates on multiple theological levels in the NT. John 10:10 sets the starkest contrast in Scripture: 'The thief comes only to steal and kill and destroy; I have come that they may have life, and have it to the full.' Jesus identifies the devil as a kleptēs — a stealth operator who takes what does not belong to him: identity, joy, purpose, eternal life. John 10:1 identifies false teachers as kleptai — those who enter by the wrong way, not through the door (Christ). The Day of the Lord is described as coming 'like a thief in the night' (1 Thess. 5:2; 2 Pet. 3:10; Rev. 3:3; 16:15) — not evil like a thief but unexpected, sudden, without warning. This language demands watchfulness and readiness. Jesus Himself uses the image in Revelation 3:3 and 16:15: 'Behold, I come like a thief. Blessed is the one who stays awake.'