According to the Liddell and Scott Greek lexicon, the word σημεῖον (derived from the older word σῆμα) was used in reference to conventional signals, ‘signs’ from the gods (omens or portentous events), and elements of reasoning (‘proofs’). The usual sense of σημεῖον in the New Testament, where it occurs frequently, seems to combine the ‘proof’ sense with the sense of ‘signs from the heavens.’ But the NT frequently warns us to beware of false prophets bearing great ‘signs’! In Matthew 24, the disciples of Jesus ask him what will be “the sign of his coming and of the close of the age” (σημεῖον τῆς σῆς παρουσίας καὶ συντελείας τοῦ αἰῶνος). His answer takes most of the chapter and includes various cataclysmic events; he also says (24:24) that ψευδοπροφῆται (false prophets) will arise (ἐγερθήσονται, the same verb used for resurrection!) and give great signs (καὶ δώσουσιν σημεῖα μεγάλα). But then will appear the sign of the son of humanity in heaven (24:30, φανήσεται τὸ σημεῖον τοῦ υἱοῦ τοῦ ἀνθρώπου ἐν οὐρανῶ), the true sign.
And how does one know the true sign from the false? We don’t know the ‘day and hour’ when the Son of man will come, but according to the gospel, the coming will be as undeniable and irresistible as the flood of Noah’s time that swept everyone away (Matthew 24:39). In that light, how do we read the statement of Jesus to his contemporaries that ‘this generation will not pass away till all these things take place’ (Matthew 24:34)? The ‘proof’ of that theorem is left to the reader.