In "Evangelicals, Heresy, and Scripture Alone" Mathew Block discusses why it is that so many Evangelicals hold views the Church has long considered heretical particularly about the person of Christ and the person-hood of the Holy Spirit. A possible answer:
...[M]any Christians seem to think saying Sola Scriptura is the ultimate authority somehow means it is my personal “solo” reading of Scripture that is authoritative. They reject the witness of the Church down through the ages in favor of a personal, private understanding of Scripture (which is not at all what the reformers meant by the term “Scripture alone”). ....
.... In other words, Evangelicals believe the Bible is authoritative; and that authority is mediated by individual believers, rather than the church (even though the Bible explicitly says that authority is to be exercised by the church—e.g., Matthew 18:15-17, 1 Corinthians 5:11-13, Titus 3:10-11, etc.)
If we are going to address the rise of heresy in our churches, then Christians must rededicate themselves to reading the Bible in community—with the local church, yes, but also with the Church throughout history. If the Bible is truly the authority Evangelicals say it is, then we must also recognize that God has exercised that authority over Christians other than ourselves. The history of the Church, in its creeds and confessions, is a witness to other Christians who have been shaped by and wrestled with the Word of God.
If instead we ignore the ways in which the Church has expressed its beliefs—if we ignore the ways in which God has shaped the faith of the Church historic through His Word—then we are really denying that the Scriptures are authoritative at all. We are in effect saying that we do not trust God’s Word to have acted on any Christians other than ourselves. Instead, we are elevating ourselves—our own hearts—as the ultimate judge, both over Scripture and the God who has shared that Word with the Church down through history. And that is heresy of the highest degree. .... [more]