Jesus came as the Jewish Messiah of Judaism in fulfillment of Jewish prophecy and he sent a prefect example for us to follow of how to practice Judaism by walking in sinless obedience to the Torah, and we are told to follow his example (1 Peter 2:21-22) and that those who are in Christ are obligated to walk in the same way he walked (1 John 2:6). While Gentiles do not need to convert to being Jewa in order to follow Christ, it is contradictory for a Gentile to want to be a follower of Christ while refusing to follow his example.
In Acts 21:20, they were rejoicing that tens of thousands of Jews were coming to faith who are all zealous for the Torah, which is in accordance with believing in what Jesus accomplished through the cross (Titus 2:14), so Jews who were coming to faith in Jesus were not ceasing to practice Judaism in accordance with his example. This means that there was a period of time between the resurrection of Jesus and the inclusion of Gentiles in Acts 10 that is estimated to be around 7-15 years during which all Christians were Torah observant Jews, so Christianity at is origin was the form of Judaism that recognized Jesus as its Messiah. It is impossible to be a follower of Jesus while wanting nothing to do with being a follower of the religion that he followed.