The Old and New Testament is one book written by the Holy Spirit. The old Testament is still in effect. The Law that Jesus paid for was the Law of sin and death. Matthew 5:17-20 tells you the Law is still in effect (The 10 Commandments). Jesus said "John_14:15 "If you love Me, keep My commandments.
Jesus didn't start a new religion. You still need the Old Testament. To learn who Israel is just read Gen 48 and learn Jacob (renamed "Israel") adopted 2 Gentile children of Joseph so they could receive an inheritance in the promised land from God. There are No, none, nada, covenants made with Gentiles, you must be adopted into God's family,... Israel to receive Heaven. You are adopted by the blessing of the "cross" of Jacob in Gen 48:16-22. Jesus said "Salvation is from the Jews". You must be adopted to receive an inheritance from God.
Generally, when you read about Ephraim in Scripture it's referring to the line of Jesus as Joseph was a type of Jesus. God married the Jews at Mt Sinai when the Jews told Moses "All this we will do" when Moses told them what God said on the tablets. Jesus will marry His church when they are raptured on some future Feast of Trumpets. In the Last Days we will be joined with the Jews as in Ezekiel 37:16-28.
Jesus was born a Jew, Lived as a Jew, followed the Leviticus 23 feast Days, Observed the Sabbath on Saturday, died as a Jew, and sits on His throne in Heaven as the Lion of Judah. He will come back to rescue His people and His land in Israel in the last days. Satan's plan has been from the beginning to kill God's people and take God's land. Knowing that, choose which side to be on,...Hamas or Israel. Israel is literally your adopted brothers if you are a Christian. Hamas is from Satan and is hell bound.
Re the Old Covenant law. Theologians (Westminster Confession) make a distinction between civil, ceremonial, and moral law. The New Testament abrogated the civil and ceremonial aspects of the law, but moral law continues. Indeed, Jesus strengthened moral law. At least 9 of the 10 commandments were reiterated as valid in the New Testament.