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
Paul is not speaking of each having a "portion" of faith but rather of each having a "measuring device" of faith:
Westcott and Hort / [NA27 variants] Romans 12:3 Λέγω γὰρ διὰ τῆς
χάριτος τῆς δοθείσης μοι παντὶ τῷ ὄντι ἐν ὑμῖν μὴ ὑπερφρονεῖν παρ' ὃ
δεῖ φρονεῖν, ἀλλὰ φρονεῖν εἰς τὸ σωφρο...