How do Christians reconcile Deuteronomy 13:1-4 with their belief in the abrogation of the Mosaic law?

Deuteronomy 13:1-4 [states](http://www.mechon-mamre.org/p/pt/pt0513.htm) : >1. All this word which I command you, that shall ye observe to do; thou shalt not add thereto, nor diminish from it. > > 2. If there arise in the midst of thee a prophet, or a dreamer of dreams--and he give thee a sign or a wonder, > > 3. and the sign or the wonder come to pass, whereof he spoke unto thee--saying: 'Let us go after other gods, which thou hast not known, and let us serve them'; > > 4. thou shalt not hearken unto the words of that prophet, or unto that dreamer of dreams; for the LORD your God putteth you to proof, to know whether ye do love the LORD your God with all your heart and with all your soul. My understanding is that Christians believe that Jesus abrogated the Mosaic law (e.g. keeping the Sabbath, dietary restrictions, etc.) and they no longer feel bound by them. How, then, do they understand the above passage, which states that one may not subtract commandments from the Law, and which seems to imply that a true prophet will not do so?