answer: everything. They are separate and incompatible. "messianics" often claim they can't be Christians because they celebrate Christmas or other such nonsense. It's a dodge. Their beliefs are 100% Christian beliefs and have little connection to Judaism.
A comparison of the three "Abrahamic" religions (note where "messianic" beliefs fall)
JUDAISM/ISLAM
- says that no human can ever die for the sins of others
CHRISTIANITY (including ‘messianics’)
- says that Jesus died for the sins of mankind
JUDAISM/ISLAM
- says that all humans are born pure, and innocent
CHRISTIANITY (including ‘messianics’)
- some say that all humans are born with 'original sin'.
JUDAISM
- a “virgin birth”/offspring w/human women is a pagan myth mainstay – G-d doesn’t become human
CHRISTIANITY/ISLAM (including ‘messianics’)
- says that Mary gave birth, though a virgin
JUDAISM
- says that the Jewish Messiah won’t need a 'second coming'
CHRISTIANITY/ISLAM (including ‘messianics’)
- says that Jesus will have a 'second coming'
JUDAISM/ISLAM
- says that every human should speak directly to G-d
CHRISTIANITY (including ‘messianics’)
- Jesus claims in the 'new testament' that the 'only way' to G-d is via him and often pray to Jesus before G-d
JUDAISM/ISLAM
- says G-d is one, indivisible, cannot be separated into three aspects/incarnations
CHRISTIANITY (including ‘messianics’)
- some sects speaks of the Father, the Son, and the Holy Spirit
JUDAISM
- says that we are ALL equally G-d's children
CHRISTIANITY (including ‘messianics’)
- says that Jesus was 'god's son' above all others
ISLAM
- some Muslims claim that the prophets are perfect and have never sinned, others believe the prophets sinned and repented
JUDAISM
- has no concept of 'hell'
CHRISTIANITY (including ‘messianics’)
- *some* Christians believe non-believers go to 'hell'
ISLAM
- most Jews and Christians and all non-Jews/non-Christians go to hell though some state that only Allah knows who will end up in hell
JUDAISM
- has no 'devil', the Jewish 'Satan' is just an ordinary angel, under G-d's control
CHRISTIANITY (including ‘messianics’)
- describes 'Satan' as a devil and 'fallen' angel
ISLAM
- Satan was a jinn created from fire
JUDAISM
- the 'messiah' will be a normal, mortal man who must fulfill all the Jewish messianic prophecies in one normal, mortal lifetime
CHRISTIANITY (including ‘messianics’)
- Jesus was the 'messiah' and will fulfill the prophecies when he 'returns'
ISLAM
- believes Jesus was a prophet, not a “son of G-d”. Mohammad is the last prophet
JUDAISM
- says that the righteous of ALL faiths will reach 'gan eden' or 'garden of eden'.
CHRISTIANITY (including ‘messianics’)
- some Christians insist that only those who 'know Christ' can reach heaven
ISLAM
- some Jews and Christians will reach Paradise, Muslims reach paradise and have it better than Jews and Christians. Many Muslims believe that G-d judges all equally.
JUDAISM
- The “law”/covenant is seen as a blessing, not a burden that can never be fulfilled. Being perfect isn’t the point in Judaism. Being aware of the 613 commandments, we have G-d on our minds almost hourly. That is part of the covenant, we obey G-d’s laws and G-d ensures Judaism will survive forever.
CHRISTIANITY (including ‘messianics’)
- Views the commandments as restrictive and impossible (except the 10 in stone). It is believed in Christianity that the death of Jesus was necessary because living by the laws is impossible and view the commandments as something of a curse.
ISLAM
- Views religious laws as part of the religion and of being observant and submissive to G-d, much as Jews do.
JUDAISM
- forbids Jews from trying to convert anyone to Judaism
CHRISTIANITY (including ‘messianics’)
- believes in actively 'witnessing' and encouraging people to convert to Christianity
ISLAM
- all must convert to Islam or pay a fine to remain Christian or Jewish and live as 2nd class citizens (all others must convert or die)
JUDAISM
- The Torah remains unchanged
CHRISTIANITY (including ‘messianics’)
- differences from the original Torah
ISLAM
-claims the Torah and Christian bible were corrupted (with no proof)
JUDAISM/ISLAM
- no concept of original sin
CHRISTIANITY (including ‘messianics’)
- some sects of Christianity believe in “original sin” to justify the need for a savior. Others believe that humankind cannot keep from sinning and requires a savior to cleanse them.
Thanks to MP with some twists by QP and me