Where does Yahweh draw the line between Christianity and Judaism, specifically in Acts 15?
Yahweh made a covenant with Israel only. Others not of Israel can be grafted into that covenant promise, however, as Paul explains in Romans 9 and 11. The issue in Acts 15 was whether the new convert had to first convert to Judaism through circumcision before becoming a believer in Messiah. James explains in verses 19-21 that for now they must quit their worst heathen religious practices. They would in time learn the laws given by Moses as they came together each Sabbath day. They did not need to convert to Judaism first as the Jewish religious leaders were telling them.
