Sooner or later thoughtful people will ask this question. The Old Testament, or Hebrew scriptures, can be viewed as the spiritual history of the human race. After the first ten chapters of the Book of Genesis, this history focuses specifically on one community, the people Israel.
We follow their story through good times and bad, when they prove faithful and especially as they are spectacularly unfaithful to their destiny as the people of God. The story includes families that fail to love one another; leaders who lead their people into hellish circumstances; prophets who speak for God in an attempt to avert catastrophe; wise ones who teach the truth; and brave ones who do what's right despite the cost. In other words, it's a familiar story that recurs in every generation.
The New Testament, or Christian scriptures, can't be appreciated apart from this earlier saga. For example, Isaiah foretells the birth of a Prince of Peace; the gospels announce the birth of Jesus. Symbols or types contained in the Old are re-presented in the New (the Ark of the Covenant reveals and conceals the Divine Presence in precious materials; Mary of Nazareth bears the Son of God in her own body).
Perhaps the most compelling illustration of how the Testaments fit together is the relationship between the Creation story in Genesis and the opening of the Gospel of John. "In the beginning . . . God created the heavens and the earth," Genesis intones. "In the beginning was the Word, and the Word was with God, and the Word was God," John declares.
In both of these beginnings a world springs to life as a result of the Divine Word. In each instance God's Word assumes material form and roams about. In Genesis the glorious beginning winds up in tragic defeat as humanity falls into sin and becomes vulnerable to suffering and death. In John the rather humble beginnings of God's-Word-among-us lead to triumphant reversals: Sin is defeated and death loses its sting in redemption.
Throughout the twin stories of the Testaments we hear about God's mercy and love and the constant striving of God to rescue us from our self-inflicted misery. As the rabbis say, the whole Bible can be boiled down to four words: We sin. God saves. Those last two words transform our history into the history of salvation.
Isaiah 9:6-7 and Luke 2:1-14; Exodus 37:1-9 and Revelation 12:1-5; Genesis 1:1-31 and John 1:1-14
The Jewish People and Their Sacred Scriptures in the Christian Bible by the Pontifical Biblical Commission
Invitation to the Old Testament and Invitation to the New Testament by Alice Camille (ACTA Publications, 2004)
Biblical Literacy: The Essential Bible Stories Everyone Needs to Know by Timothy Beal (HarperOne, 2009)