Epeiper appears only once in the New Testament (Romans 3:30), combining epei (since) + per (indeed/precisely) for an emphatic causal assertion. Paul uses it to ground the unity of Jew and Gentile justification: 'since there is only one God [epeiper], who will justify the circumcised by faith and the uncircumcised through that same faith.' The word is the linchpin of one of the most compact theological arguments in all of Scripture.
Romans 3:29-30 is a tour de force of monotheistic logic. The question 'Is God the God of Jews only?' is answered with epeiper — 'since indeed' there is only one God, He must be the God of all peoples. The oneness of God demands the universality of the Gospel. Epeiper thus connects the Shema ('Hear O Israel: the LORD our God, the LORD is one,' Deuteronomy 6:4) to the missionary imperative — one God, one way of salvation (by faith), one family of God spanning every ethnicity. The same logic powers Galatians 3:28 and Ephesians 2:14-16.