How did Christianity become so dramatically different from Judaism?

Well, Christians would tell you that God taking human form and creating a new religion is what changed it. The Law and the Prophets were completed, and something new was generated in its place. Judaism is arguably obsolete, from a Christian perspective.

Rabbi Samuel b. Nahman said: At the time that the Holy One, Blessed Be He created Man, He created him as an Androgynos.

Resh Lakish said that at the time that [Adam] was created, he was made with two faces, and [God] sliced him and gave him two backs, a female one and a male one, as it says And He took from his sides,[2] as it says, And to the side of the Tabernacle.[3]

R. Berachya and R. Chalbo and R. Samuel b. Nahman said: At the time that the Holy One, Blessed be He created man, He created him from one end of the earth until the other, filling the whole world. He created [Man] from the east to the west. From where do we learn [that man was created from the east to the west]? As it says, You have fashioned me behind and of the earth to the other]? As it says And from one edge of the heavens until the other edge of the heavens.[4] And from where do we learn that before.[5] And from where [do we learn that man was created from one end Adam filled the space of the whole earth? As it says, And lay your hand upon me.[6]

>>2801264
But confused-kun,Christianity IS Judaism. Its correct Judaism the way it's supposed to be practiced.

279 "In the beginning God created the heavens and the earth."116 Holy Scripture begins with these solemn words. The profession of faith takes them up when it confesses that God the Father almighty is "Creator of heaven and earth" (Apostles' Creed), "of all that is, seen and unseen" (Nicene Creed). We shall speak first of the Creator, then of creation and finally of the fall into sin from which Jesus Christ, the Son of God, came to raise us up again.

280 Creation is the foundation of "all God's saving plans," the "beginning of the history of salvation"117 that culminates in Christ. Conversely, the mystery of Christ casts conclusive light on the mystery of creation and reveals the end for which "in the beginning God created the heavens and the earth": from the beginning, God envisaged the glory of the new creation in Christ.118

>>2801264
Because Paul, not any of the 12 apostles, was the guy who spread Christianity the furthest, and he had most of his career in the Greek speaking world. Not surprisingly, Christianity as it became the mainstream adopted an enormous amount of Greek theological and spiritual ideas, moving away from its Jewish roots.

>>2801294
Judaism had been Hellenized by the time Christianity first appeared. There are mosaics on synagogue floors from the period with the zodiac on them and plenty mingling of Judaic and Platonic thought which set a precedent for the Gnostics. It was later Pharisee derived rabbinical Judaism that demanded more strict observance or interpretations of Judaic codes as a counter reaction the practices of Christianity.

I imagine one of the reasons Christianity spread was because it preached that anyone could be saved, even the lowliest peasant, and the afterlife and redemption wasn't exclusive to Roman elites or jewish heritage.

>>2801371
Sure, Judaism had Hellenic influences, but "mainstream" Judaism had nothing like what Christianity had, like blurred lines between Gods and men, Gods procreating with humans, death and resurrection motifs for salvation, or sacred and secret rites of initiation.

> It was later Pharisee derived rabbinical Judaism that demanded more strict observance or interpretations of Judaic codes as a counter reaction the practices of Christianity.
Pharisaic Judaism precedes Christianity by centuries, and was the single largest sect by the time of the Jewish wars if we believe Josephus, when Christainity was still a tiny nothing cult. Claiming that Phariseeism is a reaction to Christianity is just stupid.

>>2801264
Influence from other culture.

Just like how Judaism became so dramatically different after contact with Persian culture, so too did the Christians with Greek/Roman culture.