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Also, what was the faith when the New Testament didn't exist yet? It was passed down faithfully via oral tradition.

Please read your Bible.

Jesus laid out what the Old Testament was before the resurrection. Peter recognized Paul's writings as inspired. Paul recognized Peter's writings as inspired. All the books of the New Testament were written before the last Apostle died.

IOW, the Scriptures were provided to us before the end of the Apostolic Era so that we would not fall into the same error as the Jews who had so grossly manipulated the law by their "tradition".