Final answer:
The Old Testament leads to Jesus, who Christians see as the fulfillment of messianic prophecies and typological parallels, establishing a spiritual rather than political salvation which is the basis for the New Testament's inclusion alongside the Old Testament.
Step-by-step explanation:
The Old Testament is seen by Christians as leading to Jesus, the Messiah, through the fulfillment of prophecy, typology, and the establishment of a new Covenant.
Prophecies in the Hebrew Bible, which suggested a Messiah would liberate the Jews and fulfill God's Covenant, are believed by Christians to have been realized in Jesus Christ.
These prophecies, coupled with typological foreshadowings, such as Abraham's sacrifice of Isaac mirroring Christ's sacrifice on the cross, are seen as indicative of God's plan for salvation.
Early Christians, following Paul, came to view Jesus' mission as a spiritual liberation rather than a political one, ultimately leading to the inclusion of both the Old Testament and the New Testament in the Christian Bible.