He was most probably rendering "eis" the same way every time. He probably translated "en" as "in" every time too.
That's nice, when you can't read the original but you want to know what word might be used here or there. Prepositions, however, are extraordinarily flexible. It isn't enough to know what word is present, but the sense in which it is used. That requires the translator's art. It isn't a plug-and-play exercise. A good quality translation tries to convey--as nearly as possible--both the literal words and their meaning.
"Believing in" or "having faith in" is by now a pretty plainly understood idiom. It defines the object of our trust. "Believing into" is a new phrase, poor English, and doesn't clarify but confuses. You can have confidence in your ordinary translation. And if you like, use Green's or Young's or another to supplement your study.
