Arguing with missionaries is fun, until you meet one who knows how to read hebrew and understands it. It is very important to understand tanach and have a good hebrew vocabulary to be able to debate them. Otherwise they will make mincemeat out of you. If not for the time I used to volunteer with anti missionary groups I would never have learned most of sefer melachim, yeshaya, yirmiyah, tehillim and much of trei asar.
Maybe these groups exist so that yeshiva guys are forced to learn sifre nach. Just a thought 🙂