At its origin, Christianity was originally a sect of Judaism. Jesus did not come to start his own religion, but rather he was born a Jew, raised a Jew, became a Jewish rabbi, had Jewish disciples, is the Jewish Messiah, fulfilled Jewish prophecy, will return as the Lion of Judah, and came to bring fullness to Judaism. Stephen was falsely accused of teaching Jews against obeying God's Torah (Acts 6:13) and Paul took steps at the direction of James to disprove false rumors that he was teaching Jews against obeying God's Torah and to show that he continued to live in obedience to it (Act 21:20-24), so if no one in leadership was teaching Jews against obeying God, then all Christians were Torah observant Jews for roughly the first 7-15 years after Messiah's resurrection up until the inclusion of Gentiles in Acts 10, and this has always been the way that Christianity has been meant to be followed in accordance with the example that Messiah set for his followers in obedience to the Torah, but with the inclusion of Gentiles. Over time, Judaism and Christianity became distinct, with Judaism teaching to follow God's Torah, but not their Messiah, with Christianity teaching to follow the Messiah, but not God's Torah, and with both teaching half the truth. The Torah is the way (Deuteronomy 8:6, Jeremiah 6:16-19, Psalms 119:1), the truth (Psalms 119:142), and the life (Deuteronomy 30:15-20, Proverbs 3:18, Matthew 19:17), the Messiah is the way, the truth, and the life (John 14:6), the Torah is God's Word, and Messiah is God's Word made flesh, so there is no following one apart from following the other.
Replacement Theology is teaching that Gentiles have replaced Israel in the economy of God, which is no according to what the Bible says, but what is according to the Bible is that Gentiles have become grafted into Israel. According to Ephesians 2:12-19, Gentiles were once separated from MEssiah, alienated from Israel, strangers to the covenants of promise, having no hope, and without God in the world, but through faith in Messiah, all of that is no longer true in that Gentiles are now no longer strangers or aliens, but are fellow citizens of Israel along with the saints of in the household of God. According to Romans 9:6-8, not all who are descended from Israel are Israel, but rather Israel is made up of those who have faith in the promised Messiah. According to 1 Peter 2:9-10, Gentile are now included as part of God's chosen people, a royal priesthood, a holy nation, and a treasure of God's own possession, so through faith in Messiah, Gentiles have become joined to Israel. As I stated, the Septuagint uses the same word to refer to the Church of Israel in the wilderness, so Israel is the Church.