Yep. It would be called a false prophet. :)
Now, if all the details about him matched thousands of years of prophesy and you got to watch him perform routine miracles on a daily basis for several years, then it would be your responsibility to recognize him and get on board.
A cult, you mean. Specifically of the 'end of the world' variety.
They don't. You were misled, Jesus actually satisfies almost none of the criteria for messiah laid out in the Torah: http://www.aish.com/jw/s/48892792.html
Muhammad also performed miracles. Is Islam therefore true? "Well hang on" you might say, "the only record of those miracles is the Qur'an, hardly an unbiased source". But the same is true of Jesus' purported miracles. They are recorded in the Bible, but no other source from that time period.