(1) PERIPHRASTIC FORM OF THE FUTURE. A Future tense composed of a Present Participle and the Future of the verb eivmi, is found occasionally in the New Testament. The force is that of a Progressive Future, with the thought of continuance or customariness somewhat emphasized.
Luke 5:10; ἀνθρώπους ἔσῃ ζωγρῶν, thou shalt catch men, i.e. shalt be a catcher of men.
Luke 21:24; Ἰερουσαλὴμ ἔσται πατουμένη, Jerusalem shall [continue to] be trodden under foot.
(2) Μέλλει with the Infinitive is also used with a force akin to that of the Future Indicative. It is usually employed of an action which one intends to do, or of that which is certain, destined to take place.
Matt. 2:13; μέλλει γὰρ Ἡρῴδης ζητεῖν τὸ παιδίον τοῦ ἀπολέσαι αὐτό, for Herod will seek the young child to destroy it.
Luke 9:44; ὁ γὰρ υἱὸς τοῦ ἀνθρώπου μέλλει παραδίδοσθαι εἰς χεῖρας ἀνθρώπων, for the Son of man is to be delivered up into the hands of men. See also Matt. 16:27; 20:22; Acts 5:35; 20:38; Rom. 8:13.
(3) By the use of the Imperfect of me,llw with the Infinitive it is affirmed that at a past point of time an action was about to take place or was intended or destined to occur.
John 7:39; τοῦτο δὲ εἶπεν περὶ τοῦ πνεύματος οὗ ἔμελλον λαμβάνειν οἱ πιστεύσαντες εἰς αὐτόν, but this spake he of the Spirit which they that believed on him were to receive. See also Luke 7:2; John 6:71.