I havent studied her books enough to say if they contain unbiblical concepts or not.

I would say Peter, Paul and John where inspired to write what would be "standard of character, test of experience, authorative revealer of doctrine and a trustworthy record of God's acts in history."

Paul wrote of prophets to come:

1Co 14:37 If any man think himself to be a prophet, or spiritual, let him acknowledge that the things that I write unto you are the commandments of the Lord.

and

1Co 14:3 But he that prophesieth speaketh unto men to edification, and exhortation, and comfort.

Why do you believe Peter, Paul, and John's epistles contain inspired insights?

Peter and John walked with Jesus for 3 years and then the rest of their lives with the Spirit. Paul as we know meet Jesus and was choosen as an apostle by Jesus. The bible witness is in their favour.
There are historians who have studied the historicity of the biblical books. I have read one of them that was conserned about the NT books. The author made a good cause for their authenticity(sic).
The unison of voices from church history testify in their favour.
But ultimately it is faith. I choose to believe that all these witnesses are telling the truth about these three people and I choose to believe that they where choosen by God to bring the truth about Him to all who would listen.

Why do you believe they wrote inspired epistles/gospels?

I believe they are inspired because I want to believe. I choose to believe. The same thing applies to the SOP. I beliee she is inspired because I want to. Yes, there is evidence to support my belief. But in the end it is because I want to believe.

The question isn't about inspiration as both the Bible and the writings of EGW are inspired.

The question isn't about inspiration, it's about authority.

All other inspired writings are to be examined by the Bible.

Further still, even the New Testament is to be examined by the Old Testament. Even Paul stated that to the Bereans.

But why? Who said so? Why do you believe the OT is inspired and authoritative? Doesn't it boil down to faith? You want to believe. You choose to believe. The fruit of faith feels good. It rewards and satisfies.

Christ, for One said so about the only Scriptures that existed back then.

Mike is bringing up a good point. The Scriptures nowhere say what the Scriptures are. This has come down to us by tradition. We read what tradition has said is inspired, and come to a conclusion as to whether it is inspired or not. We don't make this decision based on authority, because there is no authority to base the decision on until *after* we believe. So we make the choice to believe, based on whether we think we should or not.

Similarly with Ellen White. There's nothing in Scripture which tells us to believe in Ellen White's writings. Now we can compare them with Scripture, and should, to see if they agree or not, but that's not how people make the decision to believe in her. It's because there is something special about the writings which bears witness to our spirit that her words are not just human words. We sense the Spirit of God is involved, and choose to believe. We can't believe on the basis of authority, because there is no authority which tells us to believe her writings, outside of her writings themselves.

Amen, Tom. Reading what Jesus said about the OT requires faith, too. I choose to believe the NT writers quoted Jesus accurately, or close enough. There are as many Bible believing religions in the world as there are stars in the sky (not really). Each one believes their version or translation of the truth is right and correct. So, I have come to the conclusion that the SOP was necessary to get us on track and to keep us on track. Without it, we are no better off than any other religion.