Sometimes Jesus is in vogue, and this happens in different ways. I am not talking about the History Channel around Christmas and Easter, but rather about ethics and liberal Christianity. There are a great many number of people who pit the Gospel accounts over against the rest of the Bible, and who likewise believe that every ethical command of God must be derived from the Sermon on the Mount. (I will get to the deep irony of this in a minute.) When Jesus speaks in the Gospels, the thinking goes, what He says trumps everything else in the Bible. Moreover than that, it is perfectly acceptable to shrug off the entire Old Testament as “ridiculous” or even “barbaric.” After all, we have New Testament Gandhi Jesus.
The problem with all of this is very simple: in order to believe in this Jesus, you must actually disbelieve what Jesus in the Gospels says. At this point, you simply cannot have your cake and eat it too. If what Jesus says goes as the rule of life for Christians, than the Old Testament comes with it.
Two Sayings in Matthew
The height of all irony in this view comes from something Jesus actually says in the Sermon on the Mount itself. Let me paste it here for you, but you may have to look it up for yourself anyways.
“Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil. For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.” 
My plan here is not to do a full exegesis of this passage.  I really just want to point out the fact that most of the folks I described above would have an awfully hard time agreeing with Jesus here. Until heaven and earth pass away? Jesus meant that no jot or tittle would pass away until we have iPhones. This must be what He meant!
The second passage I would like to point out comes right before the Sermon. Here Jesus, when resisting the wiles of the Devil, says this, “It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God.” Where is this written, Jesus? Deuteronomy. God’s Law. What Word is Jesus living by here? God’s Law. That’s from the Old Testament.
I need not belabor the point. You simply cannot extract Jesus and set Him over against the rest of the Bible. In order to do that, you must set Jesus against Himself.
Marcion lost to orthodoxy. He lost to Christianity.