Notice the Apostle Paul doesn't say just the New Testament or just the Old Testament. He says ALL scripture is God-breathed. There are some who are want to remove the Old Testament entirely from the Canon of Scripture, but this is impossible to do. In order to understand the Word of God in it's entirety, one must consider both the OLD and the NEW. There is a very famous saying by St. Augustine which states "The Old (Testament) is in the New revealed; the New (Testament) is in the Old concealed." Indeed the Old Testament speaks volumes (foretells) about the coming Messiah (Christ) in the book of Isaiah, in the book of Daniel and elsewhere, and the New Testament is full of references to the Old Testament in terms of Mosaic Law and scripture and to characters in the Old Testament. Indeed without the understanding of the Torah and the Old Testament, the New Testament reader would be lost. It would be like entering a movie theater showing Star Wars in Intermission and wondering who Luke or Leia or Hans Solo or Yoda or Wookie or any of the other characters were because you missed the first half entirely....! This is the reason why you study both Hebrew and Greek and read the scripture in the two original languages in Seminary. In order to understand scripture, you must have a good understanding of both OLD AND NEW ---- The Complete (ALL 66 Books of the) Bible.