You might have heard the phrase "replacement theology" uttered by some preacher somewhere, or read it on the Internet, wondered what it meant, or if it was important. The official word for this doctrine is supercessionism. It is the idea that the Christian Church replaced Israel in God's plan, after Christ was crucified. In the past, both ancient and more recent, it has been used to justify anti-semitism, which is devilish.
Supercessionism denies that God has any special prophetic intentions for the Jews anymore. There are Presbyterian preachers and writers whose doctrinal views of Israel are supercessionist. They protest the phrase "replacement theology", but the substance of what they preach amounts to it.
How do we know that God didn't replace the Jews with Christians? Paul in Romans 11 says forcefully that God has not completely cast away the Jews (11:1). God elected Jews in eternity past, and He continues to redeem a Jewish remnant by His grace (11:2-5). Paul knew that not all his fellow Jews were irrevocably hardened (11:13-14).
Each individual Jewish person's unbelief toward Christ meant they were cut off from the tree of Abraham (11:17-19). Paul meant Jewish people as individuals, not the whole nation. If an individual Jewish person repented, and trusted in Christ, God would "graft" them like branches back in (11:23-24). God has certainly not rejected the Jews absolutely or irreversibly.
There is only a partial hardening of the Jews against Christ now, until God finishes the age of Gentile redemption (11:25). The Lord permits many of them to go their way. But, in the future, God will miraculously turn ungodliness away from the Jews, and take away their sins (11:26-27). Zechariah predicts this in Zechariah 12-13.
We know that "Israel" in verse 26 doesn't mean the Christian Church, because these Israelites are ones who are enemies of the Gospel (verse 28). God still loves the Jews, because of His love for and covenant with Abraham, Isaac, and Jacob (11:28b). God's ancient call to the Jews to be His people, and the covenant gifts He gave them, cannot be revoked (11:29).
Covenant theology does not automatically lead to replacement doctrine. Covenant theology says that the three Persons of the Trinity covenanted with each other in eternity to accomplish salvation by grace (the word "covenant" being used in a more broad sense of mutual agreement).
Each of God's six historical covenants -- Adam, Noah, Abraham, Moses, David, and Christ -- fit into, and move forward, God's eternal grace-covenant. Covenant theology doesn't nullify God's past pledges to Abraham in Genesis, and it doesn't by nature say that God has no distinct plans for the Jews. There are some covenant preachers who do say such things, but I think that is the result of a reactionary spirit against Dispensationalism, or wrong teaching.