Prior to the invention of the printing press, Bibles were extremely rare and only affordable to the wealthiest of people. They all had to be written buy hand, it was not just a matter of going out and buying one. Because there were so few books, most people didn't bother to learn how to read. (To put it in more contemporary terms, most of us don't know how to fly planes; but if you could buy a plane for $25000, a lot more of us would learn.)
So, based upon the FACT that reading the Bible and using it as the sole rule of faith WASN'T EVEN POSSIBLE for fifteen centuries. I HAVE TO ASK WHY OUR LORD WOULD DEVISE A METHOD OF SALVATION THAT WAS IMPOSSIBLE FOR NEARLY EVERY PERSON.
He didn't. People have received salvation through Christ alone throughout the ages. God alone gives salvation. He draws those to Himself those who will be saved.