Everyone is under authority. Everyone. Well, everyone but God, I suppose. The question is what authority? What I'm considering here is what authority in matters of the faith?
I know a lot of people calling themselves Christians who are under their own authority. Their Christianity is formed by their own sense of right and wrong, their own sense of good and bad, their own sense of fair. If you say, "You know, the Bible says ..." and contradict their view, they'll look at you like you're crazy. What difference does the Bible make? Oh, sure, they don't say that ... because they're Christians, right? But that's what it comes down to. So they read their Bibles by the light of their own standards and come to their own conclusions. "This part is myth. That part is legend. Most of this section over here is metaphor -- like most of the Old Testament. You can't take this thing at face value, you know." And they serve as their own ultimate authority.
I know others who are bound to their Bibles. I will exaggerate to make the point. "It says Jesus is the door, so we believe He is an actual door, with latch and hinges." There is no room for context, for literary style, for variation. Many of these are the "KJV Only" types where it's not merely a woodenly literal version that they demand, but the King James woodenly literal version. It doesn't matter if it makes sense. It doesn't matter if it contradicts itself. It says it; that's the end of it.
I know some who are careful to follow their particular teachers. Maybe it's some Pentecostal favorite or some Word of Faith preacher. Almost always it's someone who tells them what they want to hear. Good things. Pleasant things. "God will make you healthy and wealthy." Things like that. Because it sounds so pleasant. These teachers will pull it from skewed texts and personal revelation. "God told me so." For these people their teachers are their authority. To be more clear, their appetites are their authority and their teachers feed those appetites.
All of us operate under some authority in our Christianity. Maybe it's a favorite teacher. Maybe it's what we were brought up with. A lot of people only recognize themselves as the authority. They do what seems good to them. All other authorities are subject to this highest authority. And while we all suggest our highest authority is God, I think that remains to be seen. If that teacher or that upbringing or our own preferences serve as the authority, can it really be counted as God? If my final authority is my own thinking and judgment, who is my final authority? My own thinking and judgment.
We need to be careful as believers. We have one ultimate authority -- God. He has spoken in His Word in a miraculous and unparalleled way. No skillful reasoning or special revelation or alternate authority claim can match up to the God-breathed Word (2 Tim 3:16-17). Any other authority is less. It's important that we learn to rightly handle His Word and rightly submit ourselves to His Word. He is the authority, not our history and upbringing, our teachers, or even our own clever minds. When we substitute anything for God and His Word, we substitute an idol for God.