New Testament theology is the study of what God has revealed about Himself in the New Testament. The system of New Testament theology takes the various truths that the New Testament books teach us about God and presents them in an organized fashion. The New Testament discloses the coming of the Messiah predicted in the Old Testament (Isaiah 9), the rejection of the Messiah by Israel, the fulfillment of the Law, the birth of the New Testament church (the body of Christ), the church age, the gospel of Jesus Christ, and instructions for believers in Jesus Christ.
The phrase new covenant (or new testament) was spoken by Christ at the Last Supper (Luke 22:20). Paul cited the new covenant as the substance of the ministry to which he was called (2 Corinthians 3:6). The Old Testament is the record of the calling and history of the Jewish nation, and it contains the Mosaic Law, the Old Covenant to which Israel was bound for many years. The New Testament deals with the history and application of the Christ’s redemption from the Law (Galatians 4:4–5), a redemption He provided through His death on the cross (Ephesians 1:7). Being the New Covenant, it supersedes the Old (Hebrews 8:6, 13).
Theology is the study of the doctrines of the Bible, following the progressive revelation that God made to man from the beginning of time to the end of the book of Revelation. New Testament theology primarily deals with the study of Christology, ecclesiology, and soteriology. Plus, it furthers our understanding of eschatology, pneumatology, angelology, hamartiology, and other fields of doctrine.