It became "not just Judaism" once non-Jews started to join, which is something that happened quite early on with guys like Paul believing that the salvation offered extended to everyone, not just Jews. But even without that, you would have the Jews who believed Jesus was the Messiah, and those who didn't.
there is no difference. Jesus was a Juda as Moses was a Christian.
at the core of every religion, when they are at their uncorrupted states, they are the same. we say that since İslam is the only religion that is uncorrupted, every member of a religion with their core teachings is a Muslim.
thats why Jesus, Moses, Abraham, Joseph and all the prophets troughout the history are Muslim.
and before you start bashing me with your civil colonist half militant nationalism, i need to remind you that people are corrupted, but the basic teaching of İslam -the Kuran itself- havent been corrupted. so the pure religion in Kuran is the purest form of religion.
so, İslam = core Christianity = core Juda = religion of Moses = religion of Jesus
you might ask why did Christianity come after Judaism if they were both the same, i will have to tell you that 1- one religion doesnt come after the other, the same religion comes again two times and 2- the reason that the religion had to repeat itself was 2a- the core of the religion was not accessible anymore 2b- people had forgotten the religion
That is because Jesus installed Christians. The first Christian congregation was formed in 33 A.C. His mother, Mary and all of his disciples converted into a Christian (you can read this in the book of Acts). Why was it necessary for Christians to appear? Well, the main reason is because Gods chosen nation: the Jews disobeyed him many times, even killing Jesus. By insuring Christians God gave all nations the opportunity to serve him. Off course there are more details, but I wish you find this little info hopeful. )
Because the first Christians were Jews. During Jesus' lifetime, Jesus never said that he was not a Jew. He in fact said in The Bible that he did not come to change the law, i.e. the laws of Moses and The Old Testament. But later after his lifetime, the Christians became a separate religion due to differing theological doctrines. Additionally, most Jews did not accept Jesus as the Messiah.
The jews persecuted Jesus and belived him to be a servant of the Devil because he could banish demons. Christians are those who follow the teachings of jesus aswell as the jewish scriptures. If you read acts, you start to see christianity branch off as Christians start to eat animals that Jews wont and such other things.
There's more difference between Judaism and Christianity than Jesus and dietary laws. Jews don't believe in hell, at least, not as an eternal place of punishment. They don't believe Satan rebelled against God, nor could even want to since angels don't have free will unlike humans. They believe Satan works as God's prosecutor to test the worthiness of mankind.
That's just a couple examples of some pretty significant differences.