Briefly, all that the prophets wrote in the Old Testament, that is, in the First Covenant, they wrote before the birth of the Messiah. But all that is in the New Testament, or the New Covenant, they wrote it after the birth of the Messiah.
The message of the prophets of the First Covenant was ‘God will send the Messiah!’. And the message of the New Covenant is ‘God has sent the Messiah, just as He promised by His prophets!’
Therefore, let us not be troubled by the fact that the Bible has two parts. Rather, thank God with all our heart that the Holy Scriptures have a first covenant and a new covenant. For in these two parts, we see that what God promised so long ago, he accomplished it! God has sent us a Saviour, as He promised to our ancestors in the Tawrat, Zabur, and the Writings of the Prophets.
Just as a seed sprouts and grows, the leaves come and the flower, before finally there is fruit. So the seed of the gospel is in the first book of Moses. The following books develop the same theme of the promised Messiah. The fruit comes in the New Testament, the Injil.
These two parts of the one book are the two lips by which God speaks of the Messiah and our salvation with one voice.