Hi Musterseed,
Ray spent a lot of time doing what very few of us do: studying the Scriptures. Really studying them, not just reading them. In some of his audios, he gives hints at how much he did this. One time, he was working on a Bible study for 14 hours (6 am to 8 pm). Other times, he would study a particular topic for several hours without getting up at all.
He also had a background as a pastor, so he already knew a lot about the Bible. Some might think that would be more harmful than helpful (having an education from a Christian college), but I think it probably helped. Even if he didn't study seriously until much later in life, it gave him the basic framework. It also would have helped him understand the beliefs of his many detractors: as he once said, he could argue their false doctrines better than they could.
The main reason, of course, is that Ray was guided by the Spirit of God. There are many Bible scholars "smarter" than Ray, who understand very little about the plan of God. Said L. Ray Smith, "I don't know anything about Hebrew and Greek!"