1)How comes Jews no longer practice polygamy?
2)In modern Judaism, can a Jew married more then one wife?
His he allow to marry more then one wife, in other words.
3)How comes messianic Jews are not consider Jews?
4)Is a Jew allow to believe as Jesus as the messiah, and Muhammad as a prophet?
In other words, what would happen if a person is traditionally Jewish, but believes in Jesus and Muhammad to be from god, would that person still be consider a Jew or no longer a Jew.
1),2) see here: http://dinonline.org/2015/11/18/marrying-more-than-one-wife-the-decree-of-rabbeinu-gershom-then-and-today
3) A person’s status as a Jew is based on either being born of a Jewish mother [or conversion]. What you are referring to is a Jew’s belief system. One of the basic principles of Jewish belief is that the Torah we received at Sinai from Moshe Rabbeinu [Moses] will never be changed in the future by anyone. The Torah explicitly warns of false prophets arising and making claims contradictory to the Torah. So while a Jew will always remain a Jew, it is certainly forbidden for a Jew to believe in Jesus or Mohammed, this for a Jew is the most severe sin of heresy and idol worship. A Jew is in fact commanded to give up his life before he accepts the divinity of these people.