1 Thessalonians 5:3

ὅταν, κ. τ. λ., when the very words, “All's well,” “It is all right,” are on their lips. ἐπίσταται, of an enemy suddenly appearing (Isocrat., Evag., § 58 ἐπὶ τὸ βασίλειον ἐπιστάς, Herod. iv. 203). αὐτοῖς, i.e., while the Day comes suddenly to Christians and unbelievers alike, only the latter are surprised by it. Christians are on the alert, open-eyed; they do not know when it is to come, but they are alive to any signs of its coming. Thus there is no incompatibility between this emphasis on the instantaneous character of the advent and the emphasis, in II., 2 Thessalonians 2:3 f., on the preliminary conditions.