A: Jeremiah 31 is the source of the term “New Covenant” The author of the book of Hebrews 10:15-17 declares that in the accomplished work of Yeshua we have the fulfillment of this prophecy.   Back in Jeremiah 17:1, the prophet reveals that what was written before on Israel’s heart:  their sin!  But according to Jeremiah 31:34b, which is quoted in Hebrews 10:17, under the new covenant sins will be forgiven and remembered no more. For believers it is already true that the Torah we follow is written in our hearts and minds by virtue of a direct personal relationship with God, that is, through Ruach ha-Kodesh  living in us. (See Romans 8).  He is our new Teacher (John 14:26). Human teachers are not yet obsolete, as we still need each other to help us heed what the Spirit is already telling us. Our understanding is not yet complete. (I Corinthians 13:12)  Of course it is not yet the case that all of Israel knows him this way.  We look forward to that day when our Messiah returns in power. (Romans 11:25-27) Meanwhile, the New Covenant is available now to as many as will receive it. (John 1:12)