The Old Testament is composed of the teachings of Moses and the prophets. The men and women of Old Testament times laid a foundation of God's word that enables us to reach Jews and Gentiles with the gospel in a much easier way than we realize. God revealed himself to the world through the Jewish people in order to prepare the world for the arrival of Jesus Christ. This is why it's important to thoroughly know the Old Testament.
John 4:34-38 shows us this truth.
“My food,” said Jesus, “is to do the will of him who sent me and to finish his work. Don’t you have a saying, ‘It’s still four months until harvest’? I tell you, open your eyes and look at the fields! They are ripe for harvest. Even now the one who reaps draws a wage and harvests a crop for eternal life, so that the sower and the reaper may be glad together. Thus the saying ‘One sows and another reaps’ is true. I sent you to reap what you have not worked for. Others have done the hard work, and you have reaped the benefits of their labor.” - John 4:34-38.