Exaiphnēs (ἐξαίφνης) means suddenly — an unexpected, immediate event. It combines ex (out of) and aiphnēs (sudden). The word marks divine intrusions and eschatological moments: the angels at the birth of Christ, Paul's Damascus road encounter, the return of Christ.
God acts suddenly. Exaiphnēs is the adverb of divine surprise — the invasion of the eternal into the temporal without prior warning. Luke 2:13: "Suddenly [exaiphnēs] a great company of the heavenly host appeared." Acts 9:3: "As Paul journeyed, suddenly [exaiphnēs] a light from heaven flashed." Luke 9:39 in the demonized boy: the spirit seizes him "suddenly." The pattern: exaiphnēs marks moments when heaven breaks through — for deliverance or encounter. The return of Christ is the ultimate exaiphnēs: "like a thief in the night" (1 Thess. 5:2), sudden, expected in its certainty but not in its timing.
Exaiphnēs appears five times in the NT, always in moments of high spiritual intensity. It stands against the routine, the predicted, the managed. The spiritual life awaits exaiphnēs — the unexpected visitation that breaks the ordinary. This is why vigilance matters: not paranoia but alertness to the fact that God can act at any moment. Advent and eschatology are both structured around this suddenly.