"'for in it God's righteousness is being revealed by reason of faith and toward faith, just as it is written: 'But the righteous one—by means of faith he will live.'"
NWT inserts "and" to help us make sense of the double prepositional phrase. It renders the genitive EK PISTEWS as "by reason of faith" and the accusative EIS PISTIN with the words "toward faith" thereby drawing a contrast between the two prepositions EK and EIS. RSV handles the phrase by using "through" and "for" in its rendering: '"For in it the righteousness of God is revealed through faith for faith; as it is written, "He who through faith is righteous shall live."'
Rogers and Rogers say that the preposition EK (in Romans 1:17) "may indicate the source of righteousness" (The New Linguistic and Exegetical Key, p. 316). Another important point to consider is that EN AUTWi ("in it" NWT) probably refers to the Gospel mentioned in the preceding verse. If that is the case and the prepositional phrase EN AUTWi is construed as an independent construction, then it would lend credence to the NIV rendering:
'"For in the gospel a righteousness from God is revealed, a righteousness that is by faith from first to last, just as it is written: "The righteous will live by faith."'
Richard A. Young thinks that EK PISTEWS EIS PISTIN "is best explained as means that is perhaps intensified by the addition of the second phrase (by faith and faith alone)." See his work Intermediate NT Greek, p. 95.