An example would be the billions of people during the 1000 years when Catholicism was the only religion and people couldn’t even read the Bible themselves.
This is a difficult one and I am not sure how to answer. I can say this though; the word of God has always been in existence, although like you say, there have been times when it was not so readily available. God tells us that His word is written in our hearts. He says; now what I am commanding you today is not too difficult for you or beyond your reach. It is not up in heaven, so that you have to ask, “Who will ascend into heaven to get it and proclaim it to us so we may obey it?” Nor is it beyond the sea, so that you have to ask, “Who will cross the sea to get it and proclaim it to us so we may obey it?” NO, the word is very near you; it is in your mouth and in your heart so you may obey it. (Deuteronomy 30:11-14)
Although we are born sinners and enemies of God, we are also born with a natural knowledge of God in our hearts. We know there is a God without being told. For those who truly search for the truth, it will be revealed to them in some way by the Holy Spirit. I think here of Martin Luther who founded the Lutheran Church. He was studying to become a monk. He had many questions and doubts about what the Catholic Church taught. He was advised by the head of the monastery to search the Bible for answers to his questions. When he searched the scriptures, he found the truth. The church opposed his sharing the truth of God’s word with others which lead him to publicly oppose the church by nailing his 95 Theses (or concerns) on the door of the Castle Church in Wittenberg (October 31, 1517), Germany for all to see.
Today the Bible is readily available to all people. It is very important that all of us search the scriptures and compare it to what our own church teaches. We need to be sure the teachings of our church do not go against what God tells us in His word. If the teachings of your church contradict what God says in His word, you need to look for a church home that teaches the truth. Jesus Himself warns us; watch out for false prophets. They come to you in sheep’s clothing, but inwardly they are ferocious wolves. (Matthew 7:15) He then goes on to say; by their fruit you will recognize them. In other words one can tell by the way people live, act and speak whether or not they love God and follow His teachings.