Evolved from Judaism

Galatians 2:14-16 / But when I saw that they were not acting consistently with the truth of the gospel, I said to Cephas before them all, “If you, though a Jew, live like a Gentile and not like a Jew, how can you compel the Gentiles to live like Jews?” We ourselves are Jews by birth and not Gentile sinners; yet we know that a person is justified not by the works of the law but through faith in Jesus Christ. And we have come to believe in Christ Jesus, so that we might be justified by faith in Christ, and not by doing the works of the law, because no one will be justified by the works of the law.    

Christianity evolved from Judaism. With very few exceptions all Paul's letters were written to meet an immediate situation. The situation being addressed in the above text has to do with a decision made in Jerusalem that in effect purported that the Jews would go on living like Jews, observing circumcision and the law, but that the Gentiles were free from those observances. Clearly, things could not go on like that, because the inevitable result was to produce two grades of Christians and two quite distinct classes in the Church.

Paul's argument runs like this. He said to Peter (Aramaic 'Cephas'), "You shared the table with the Gentiles; you ate and lived as they ate; therefore approved in principle that there is one way for Jew and Gentile alike. How can you now reverse your whole decision? You were quite willing to live like a Gentile; and now you have swung round, and you want the Gentiles to be circumcised and take the law upon them and become Jews." This did not compute with Paul.
To help better understand the context we must understand the meaning of one word in particular: sinner. When a Jew used the word sinners of Gentiles they were not thinking of moral qualities at all; they were thinking of the observance of the law. For example, Leviticus chapter 11 lays down the Jewish food laws and details and classifies the animals which they may and may not use for food. A Jew who ate rabbit, or who ate pork, broke those laws and became a sinner in this sense of the term.

So Peter would answer Paul, "But, if I eat with the Gentiles and eat the things they eat, I become a sinner." And Paul's answer would be, "We have agreed long ago that no amount of observance of the law can make a person right with God. That is a matter of grace. A person cannot earn, they must accept, the generous offer of the love of God. There is, for every believer, the temptation to try to earn the favor of God. But it is utter trust in the love of God in Christ that properly aligns a person with God. Jesus did not tell you to try to earn salvation by eating this animal and not eating that one. He told you to fling yourself without reserve on the grace of God."

Christianity that focuses too much on the belief that its law-abiding achievements are superior to that of other Christians, and that in the eyes of God they alone shall find favor, is not true Christianity at all.