Marcion was a heretic who lived in the 2nd century. He hated Jews. His belief centered around labeling the Jewish Tanakh the "old testament" and the Brit Chadasha the "new testament". These lables were designed to cast scorn on the Tanakh, citing it as old and ready to be tossed out, while the Brit Chadasha was better. The Renewed Covenant actually finds its foundation in the First Covenant. Marcion wanted that understanding destroyed.
Marcion taught that the "old testament" God had been filled with wrath and fury but the "new testament" God was merciful and loving. While scripture plainly shows Yah's mercy in the Tanakh and His wrath in the Renewed Covenant, Marcion was not interested in the facts but in rejecting the Jewish God while promoting his Greco-Roman God. Marcion was identified and rejected by most believers as a heretic.
Unfortunately, Marcion's beliefs now flood Christianity. Nearly every Bible has discarded the original names of the First and Renewed Covenant books for the Marcionic labels of old and new testament. So many Christians believe God was wrathful in the old testament but reformed His ways into a more merciful method in the new, this idea pervades even the secular world. One only has to pick up a thriller by Dean Koontz, one of the most popular authors today, to find statements about the vengeful old testament God and the merciful new testament God. The ghost of Marcion has returned with a vengeance.
It is not as if our ancestors left us in the dark. The 2nd and 3rd century believers roundly rejected Marcion and those who adopted his heresies. But as anti-semitism has crept into the "church world", Marcion has returned with it. There are believers who do not fall for this common misconception and believe the God of the old testament is really the same as the God of the new. But even they tend to believe the old testament has been made obsolete while the new has replaced it. This is a strain of Marcionic heresy that has prevented the Bride from obeying the Torah as she ought.