Icon,
Perhaps I disagree with the thinking that only believers have the law of God written upon their hearts.
I would consider that the "light" given to all is the Decalogue (John 1). All social groups whether they know the Bible or not, have similar basic laws. Some would include, murder, thievery, ... and others would include worship. That doesn't make their laws righteous, nor a statement of the but merely to point to the law being a part of the makeup of every person.
"No, God" is just as common in the darkest heart as the most illuminated.
One doesn't have to attend a dispensational church to get wrong ideas. The enemy of believers thrives in all assemblies. Isn't that the point of Matthew 13:24-30?
What Jer. and Hebrews do (imo) is reinforce each other that the covenant of promise is given, and the old is passing (fading) away. The following chapters of Hebrews shows the sufficiency of Christ and therefore the fading away of the old pictures and practices.
What some mistake (imo) is taking the fading away of the old covenant to mean the physical / social / economic Israel is replaced or no longer a viable group to be seen as ever again the chosen of God. God has always had a subset of the larger chosen. It is seen in the typical assembly - the blend of the mixture of saved and unsaved.
Is it not possible for the new covenant to extend both mercy and grace to the physical / social / economic Israel under the new covenant in the same manner it is extended to the Gentile? Is it unreasonable to see the Scriptures in the manner that those that He purposely blinded, so that the Gentiles would be grafted in, have their eyes awakened, their sensibilities to the claim of Christ born?
Does not such authority rest with the Sovereignty of God in the doctrines of Grace that we teach?
Could it be that the old theological approaches that had little or no frame of reference from which to view this modern world, are needing some modification and realignment with Scriptures as we can actually be eye witness to prophetic fulfillment the older approaches could not consider and as a result largely marginalized or assigned to the gentile church?