If so be that God is one (eiper heis ho theos). Correct text rather than epeiper. 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. 1Co 8:5; 1Co 15:15; Rom 8:9.
By faith (ek pisteōs). “Out of faith,” springing out of.
Through faith (dia tēs pisteōs). “By means of the faith” (just mentioned). Ek denotes source, dia intermediate agency or attendant circumstance.