While the Bible is a unified book, there are differences between the Old Testament and the New Testament. In many ways, they are complementary. The Old Testament is foundational; the New Testament builds on that foundation with further revelation from God. The Old Testament establishes principles that are seen to be illustrative of New Testament truths. The Old Testament contains many prophecies that are fulfilled in the New. The Old Testament provides the history of a people; the New Testament focus is on a Person. The Old Testament shows the wrath of God against sin (with glimpses of His grace); the New Testament shows the grace of God toward sinners (with glimpses of His wrath).
The Old Testament predicts a Messiah (Isaiah 53), and the New Testament reveals whothe Messiah is (John 4:25–26). The Old Testament records the giving of God’s Law, and the New Testament shows how Jesus the Messiah fulfilled that Law (Matthew 5:17; Hebrews 10:9). In the Old Testament, God’s dealings are mainly with His chosen people, the Jews; in the New Testament, God’s dealings are mainly with His church (Matthew 16:18). Physical blessings promised under the Old Covenant (Deuteronomy 29:9) give way to spiritual blessings under the New Covenant (Ephesians 1:3).
The Old Testament prophecies related to the coming of Christ, although incredibly detailed, contain a certain amount of ambiguity that is cleared up in the New Testament. For example, the prophet Isaiah spoke of the death of the Messiah (Isaiah 53) and the establishing of the Messiah’s kingdom (Isaiah 26) with no clues concerning the chronology of the two events—no hints that the suffering and the kingdom-building might be separated by millennia. In the New Testament, it becomes clear that the Messiah would have two advents: in the first He suffered and died (and rose again), and in the second He will establish His kingdom.
Because God’s revelation in Scripture is progressive, the New Testament brings into sharper focus principles that were introduced in the Old Testament. The book of Hebrews describes how Jesus is the true High Priest and how His one sacrifice replaces all previous sacrifices, which were mere foreshadowings. The Passover lamb of the Old Testament (Ezra 6:20) becomes the Lamb of God in the New Testament (John 1:29). The Old Testament gives the Law. The New Testament clarifies that the Law was meant to show men their need of salvation and was never intended to be the means of salvation (Romans 3:19).
The Old Testament saw paradise lost for Adam; the New Testament shows how paradise is regained through the second Adam (Christ). The
Old Testament declares that man was separated from God through sin (Genesis 3), and the New Testament declares that man can be restored in his relationship to God (Romans 3—6). The Old Testament predicted the Messiah’s life. The Gospels record Jesus’ life, and the Epistles interpret His life and how we are to respond to all He has done.
In summary, the Old Testament lays the foundation for the coming of the Messiah who would sacrifice Himself for the sins of the world (1 John 2:2). The New Testament records the ministry of Jesus Christ and then looks back on what He did and how we are to respond. Both testaments reveal the same holy, merciful, and righteous God who condemns sin but desires to save sinners through an atoning sacrifice. In both testaments, God reveals Himself to us and shows us how we are to come to Him through faith (Genesis 15:6; Ephesians 2:8).
The specific answer to the question on why some only read the Old testament and discard the New testament surrounds the rejection of Jesus as the Christ which is the basis of the New Testament. In this regard, the rejection of the New Testament would be those who follow Orthodox Judaism, which is the traditional religion for Jews, but it may be practiced by non-Jews as well. Not all Jews practice Judaism, of course; some reject Judaism in favor of other religions or no religion at all. Judaism is a religion that emphasizes lifestyle and values more than beliefs, so correct doctrine is not emphasized nearly as much as correct behavior. Many Jews will maintain their connection to a synagogue and continue to observe certain practices and traditions while rejecting many of the doctrines that may be taught there. One should not assume that attendance at a particular synagogue signifies acceptance of all that is taught there. Of course, this is true for Christian church attendance as well, but it seems to be even more prevalent within Judaism. A couple of centuries ago, Orthodox Judaism was the only form of Judaism. Today, Judaism is made up of three main “branches”: Orthodox (very traditional), Reform(also known as Liberal or Progressive), and Conservative, which charts a course between the other two. Naturally, there are offshoots, variations, and even hybrids of these three. Most synagogues are designated by the branch title, similar to a denominational name on a church.
Finally, there are secular or non-religious Jews (also called humanistic or non-theistic Jews) who maintain a Jewish ethnic identity through the observance of many Jewish traditions. Originally, those traditions had a religious purpose, but secular Jews attach absolutely no religious significance to them today. Secular Jews make up a significant portion of the Jewish population in the United States.
What is today called Orthodox Judaism for most of history was simply called Judaism. The term Orthodox, which literally means “right opinion,” began to be used in the late 18th and early 19th centuries to distinguish it from other approaches to Judaism that had begun to develop. Orthodox was first used as a pejorative term by more progressive Jews but came to be embraced by the traditional adherents of Judaism. Orthodox Judaism emphasizes living according to the Law of Moses (the Torah, the Old Testament), as it has been interpreted by the authoritative rabbinic tradition. According to Orthodox Judaism, in addition to the written Law, Moses also received the correct interpretation of the Law, which has been handed down by oral tradition through the rabbis until it was finally written down in the Mishnah, dating from the 2nd century AD.
Since that time, the Mishnah has been further developed and interpreted. The Mishnah and its additional histories, commentaries, and applications are known as the Talmud, of which there are two versions: the Jerusalem Talmud and the Babylonian Talmud. Orthodox Judaism considers the Babylonian Talmud to be more authoritative. The study of the Talmud is essential to Orthodox Judaism, even more important than the study of the Torah.
Rabbinic commentary on the Talmud has accrued over the years and has come to be known as the halakha. The halakha provides authoritative instruction for Orthodox Jews on religious and civil practices and is binding upon the individual and the community. Some of the distinctive practices of Orthodox Judaism include gender-segregated prayer, a refusal to travel on the Sabbath, and maintaining strict kosher observance.
There are two variations within Orthodox Judaism. Modern Orthodoxy maintains all the distinctives of Orthodoxy but freely interacts with the society at large and considers a secular education to be important. Ultra-Orthodoxy (a term that some find offensive) or Haredi Judaism tends to insulate itself from secular society, focus on religious education, wear distinctive clothing (normally black suits and white shirts for men and carefully modest dress for women), and primarily speak Yiddish. Adherents to this form of Judaism in the United States will often live in enclaves in larger cities. A subset of Haredi Judaism is Hasidic Judaism, which is then divided into various sects. Each sect within Hasidic Judaism is led by a rebbe who is believed to have direct access to God. Kabbalah, which is often described as “Jewish mysticism,” is also central to Hasidic Judaism.