Several years ago I spent some time attending a church in which people were very experiential in their faith. It was refreshing to me to see how vibrantly they worshiped and how actively they sought the mind of God for every aspect of life. It was a welcome break from some of the stoic routines I had witnessed growing up. This was not true of my parents, of course, but my church tradition didn't welcome a lot of free expression in one's intimacy with the Lord. I liked it.
As time went on though, I started to become concerned about what some people were suggesting was the "will of God" for them. I wondered how they could be so confident that God had spoken so specifically to them regarding issues I had been taught that the Bible addresses. Sure they knew that God had already spoken on their issue, and that he would never contradict his written Word later on. When I brought this up, some reacted negatively, suggesting that I was restricting the "leading of the Spirit" with the "letter of the Law." That certainly wasn't my intention, but I was concerned that they were somehow failing to recognize how much we can all be deceived by our own feelings.
Without a standardized revelation as our authority, we all are susceptible to following our own private revelation. Our own frailties and fallen natures make private revelation more susceptible to influence from our own personal agendas. When left unchecked, we can easily fall into bondage to our own fleeting emotions that we instead call "prophecies from God." How does God, in his benevolent grace, rescue us from such personally imposed dangers? Standardize revelation...The Bible.
When we fail to test what we think God is telling us against what he has already said in the Bible we can slowly develop ideas that are actually contrary to what the Bible teaches. The problem is that by that time we've convinced ourselves that God directly told us something that in actuality he has not said at all. God would not contradict his own Holy Scriptures. People can fall into this trap all the time; so can institutions.
By the end of the medieval period the church of Rome had slowly developed doctrines and traditions that actually contradicted key teachings of the Bible. To settle the conflict it was necessary to determine by what authority a conclusion would be reached to the problem. Would the Scriptures be the final authority for determining how correct or erroneous our ideas were? Or would the Pope and recent church tradition hold sway?
The conclusion reached by Martin Luther and other reformers was that the nature of man made it possible for people to migrate away from God's Word in their thinking, so tradition (though it is helpful and instructive in some cases) should not be the final authority. In addition, because human weakness is still present, the Pope should not be the final authority for determining what we must believe either. Scripture alone can hold that place. God has gracious provided us with standardized revelation that, though interpreted in community, is not subject to the community. We instead are subject to it. Sola Scriptura remains as one of the great pillars of the Protestant Reformation because we recognize God's gracious care of us in providing it as our authority for faith and life.