I, as a Jew, study the Tanach, and I have faith in what it says. Most Christians don't work that way. Christians have been told that Isaiah says that the Messiah will be born of a virgin, and afterwards, when they read the Bible, they accept that the Bible says that. It doesn't matter in the least that the Bible says 'young woman. In general, Christians do not arrive at their beliefs logically.
Christian missionaries arrive at their beliefs through faith, and often blind faith.
So argument does not really sway them. They will tell you that faith is the most important thing to have, and that we must have faith. We Jews try to tell them that what they are saying contradicts what the Torah says, and we have faith in Hashem, and in the Torah that Hashem gave us. For some reason, missionaries cannot seem to grasp this concept. I do not know why.