What were people supposed to do with an incomplete or nonexistent Bible?

If God wants everybody to be Christian and the Bible is the central text of Christianity, what were people supposed to do before it was completed, much less started? Also, is there anything on record speaking to reasons why the particular era that the Bible was written was chosen (specifically OT)? It was a gradual roll-out, so were there parts that weren't needed until they came up, or did they perhaps correlate to certain events?