Read Matthew 22:23-46.
And then read these verses from the Old Testament: Deuteronomy 6:4-5 and Leviticus 19:18.
Jesus is still under cross-examination. It's been a long day already, and we are told that it's still the same day (22:23) and now the Sadducees are trying to trip him up on resurrection. After the Pharisees hear that he has silenced the Sadducees they come back at him to ask, "what is the greatest commandment?"
He responds by quoting from The Shema, and then from Leviticus. This is the correct old (Jewish) answer - not some new (Christian) teaching. Jesus may wish to extend the definition of neighbor, but even that is not new; the Old Testament is filled with commandments about how to treat the stranger.
In the midst of all the polemics, Jesus quotes from the only Bible he had - what we Christians call "the Old Testament." He does not say, "the God of my Bible is a God of wrath but I've come to tell you about a God of mercy." Too many Christians continue to exhibit Marcionistic tendencies. Too many clergy do! It drives me nuts!
What would happen if Christians remembered that the whole of the law and prophets really are about love of God and love of neighbor; that this is the "lens" though which we are called to read and mark and learn and inwardly digest it. It's not just true because Jesus said it; Jesus said it because it's true.
Part of what I think has happened to mainline Protestant Christianity (I am in no position to speak for others) is that we've lost the prophetic voice. We've lost why it is that the disciples, when asked who Jesus was, began with a litany of the prophets. Some say Jeremiah. Others Elijah. Some think John the Baptist is back...
The Jewishness of Jesus matters, and the so-called "Old" Testament matters for those who seek to follow him today. Otherwise when we read in The Book of Common Prayer that "Jesus said..." these things about the two great commandments (see BCP 351) we may forget that he was just quoting from the Bible.