Concerning Jeremiah 31, using that as an example if dispensational error, it is grounded (it seems to me) in the tenacious belief of dispensationalists to separate Israel from the Church. "The Jewish nation is never to enter the church." -Darby
the 'basic promise of Dispensationalism is two purposes of God expressed in the formation of two peoples who maintain their distinction throughout eternity." -Ryrie
I believe that because of the belief of a distinct Israel from the Church, Jeremiah gets applied, and the New Covenant there, to the future restoration of Israel. It is very obvious to me that the New Covenant here is the New Covenant in the blood of Christ which includes all of God's people, both Jew and Gentile, as explained by Hebrews.
Jeremiah 31 is undeniably made to Israel, yet it is the Church fulfilling it. I found this quote from Ryrie:
"If the church does not have a new covenant, then she is fulfilling Israel's promises, for it has been clearly shown that the Old Testament teaching on the new covenant is that it is for Israel. If the church is fulfilling Israel's promises as contained in the new covenant or anywhere else in the Scriptures, then [dispensational] premillennialism is condemned. One might well ask why there are not two aspects to the one new covenant. This is the position held by many premillennialists, but we agree that the amillennialist has every right to say of this view that it is a practical admission that the new covenant is fulfilled in and to the church."
Well, enough on that. Dispensationalism in its core teachings does not match the teaching of the Apostles and THEIR interpretation and application of the promises of God made to Israel.
-five