The main message of the Bible can be summed up in two words: Jesus saves! This is not just the message of the New Testament, but it is the message of the Old Testament as well. Look at what Peter said to Cornelius:
Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons: 35 But in every nation he that feareth him, and worketh righteousness, is accepted with him. 36 The word which God sent unto the children of Israel, preaching peace by Jesus Christ: (he is Lord of all:) 37 That word, I say, ye know, which was published throughout all Judaea, and began from Galilee, after the baptism which John preached; 38 How God anointed Jesus of Nazareth with the Holy Ghost and with power: who went about doing good, and healing all that were oppressed of the devil; for God was with him. 39 And we are witnesses of all things which he did both in the land of the Jews, and in Jerusalem; whom they slew and hanged on a tree: 40 Him God raised up the third day, and showed him openly; 41 Not to all the people, but unto witnesses chosen before of God, even to us, who did eat and drink with him after he rose from the dead. 42 And he commanded us to preach unto the people, and to testify that it is he which was ordained of God to be the Judge of quick and dead. 43 To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins. Acts 10:34-43
Notice verse 43: To him (Jesus) give all the prophets witness. Not just some of the prophets witness to Jesus, but all the prophets. People in the Old Testament times were given the same message that we are given today: Jesus saves!