In the later verses of chapter three of Paul’s second letter to Timothy he explains that the sacred scriptures are “able to give you wisdom for salvation through faith in Christ Jesus.” 
Notice the sacred scriptures for Paul and Timothy would have been the Old Testament writings since the New Testament writings had yet to be compiled and connected to the old, thereby forming the completed bible. Yet Paul affirms that it is in the old that Timothy is to find the message of salvation through faith in Jesus Christ. H.B. Charles concluded that Timothy is instructed to “read the Old Testament text with New Testament eyes. His preaching was to focus on the divine person and redemptive work of Christ.” 
I think the charge is the same for preachers today. Although we find a more overt gospel message in the New Testament which makes for easier sermon preparation, let us not neglect the old testament scriptures. It is the Old Testament scriptures culminating in the person of Jesus Christ that were used to convert both Jews and Gentiles alike. Remember “all Scripture is inspired by God and is profitable for teaching, for rebuking, for correcting, for training in righteousness…” 
 2 Timothy 3:15
 Charles, H. B. On preaching: personal & pastoral insights for the preparation & practice of preaching Chicago: Moody Publishers, 2014.
 2 Timothy 3:16