Daniel,
“In this case, I was surprised to learn how strongly portions of the New Testament itself seems to be at odds with the early heresy that attempted to create this distinction.”
So much so that Marcion had to manufacture supposedly “original” versions of the Gospel of Luke and the Epistles of Paul stripped of all Old Testament quotations.
Even among orthodox Christians, there is a long history of theologians, preachers, and laymen regarding certain books of the Bible as more or less essential than others depending on how neatly they fit into their systematic theology.
