If so be that God is one (ειπερ εις ο θεος). Correct text rather than επειπερ. It means "if on the whole." "By a species of rhetorical politeness it is used of that about which there is no doubt" (Thayer. Cf. 1. Corinthians 8:5; 1. Corinthians 15:15; Romans 8:9.

By faith (εκ πιστεως). "Out of faith," springing out of.

Through faith (δια της πιστεως). "By means of the faith" (just mentioned). Εκ denotes source, δια intermediate agency or attendant circumstance.