The future indicative (usually in the first or third person), preceded by οὐ μή, signals a strong negation in the future.
This construction is equivalent in meaning to οὐ μή + aorist subjunctive, and probably originates from it. The change to the future indicative is accounted for on the basis both of morphological motivations (the similarity between the aor. subj. and the fut. ind.) and semantic motivations (the similarity between the plausible modality and the future tense).
This relatively rare construction, which occurs almost exclusively in the first and third persons, occurs several times in the New Testament, Xenophon and the tragic writers.
ἀλλοτρίῳ δὲ οὐ μὴ ἀκολουθήσουσιν ἀλλὰ φεύξονται ἀπ’ αὐτοῦ, ὅτι οὐκ οἴδασι τῶν ἀλλοτρίων τὴν φωνήν
They will certainly not follow a stranger, but avoid him; for they do not know the voice of strangers.