The concept of inspiration was well-known amongst ancient Hebrews and Greeks. The word theopneustos might be new, but the concept has been around for aeons, especially in Greek culture. No doubt this is where Paul, trained as a Pharisee, picked it up. He was well-read, quoting some of their poems. Consequently he would amalgamate Jewish thought with that of the Greeks. The principle of inspiration (= originating from God) is found in Jewish writings, Biblical as well as Extra-Biblical.
The commandments would be dictated by God ( Ex. 34:27 ) or inscribed by Him on the tablets (24:12; 31:18; 32:16). Jubilees refers to heavenly tablets on which not only the Law (4:5, 32) but the history, too, is already inscribed (23:32; 31:32; 32:28). The work of God in the prophets is specifically called inspiration ( Nu. 24:2–4 ; Hos. 9:7 ): “This doctrine of inspiration guarantees the authority of the Prophets and Writings in the older Synagogue, though this authority is less than that of the Law, which is given by God. While the Law was pre-existent, was taught to Moses by God, and was literally dictated or even written down by God Himself, the Prophets and Writings were inspired by God… ”
Hellenistic Judaism understood the authority of Scripture primarily in the category of inspiration. That similar ideas made their way into Palestinian Judaism, too, may be seen from 4 Esr. 14:22, where the author asks God for the gift of the Holy Spirit that he may be able to write down again just as it was everything that was in the destroyed Law and everything that took place from the beginning of the world (see TDNT, volume VI, p. 455).