However, Paul's use of Scripture is the element that is strikingly original.  In many ways, he turns Scripture on its head -- using Scripture against Scripture, and disproving the traditional understanding.  For example,  Paul quotes Romans 4:

          For what does the Scripture say? "Abraham believed God, and it was reckoned to him as righteousness" (Rom 4:3).

          Then Paul uses this verse to admit Gentiles into the covenant of faith:

          The purpose was to make him the father of all who believe without being circumcised and who thus have righteousness reckoned to them (Rom 4:11).

No one ever thought of this interpretation before.  But it is the fulcrum for Paul to move the rock of circumcision off the shoulders of the Gentiles.   Likewise, his treatment of the Law is to declare it obsolete.  

           All who rely on observing the law are under a curse (Gal 3:10),

           and in support of this revolutionary statement, he quotes from the Law itself:

           for it is written: “Cursed is everyone who does not continue to do everything written in the Book of the Law" (quoting Deut 27:26).
        
This is not a teaching sanctioned by the Law or the Prophets.  Jeremiah declared that God would one day make a new covenant, but it was Paul who "invented" the incompatibility of Law and Gospel. 

          

This innovative use of Scripture is what got him in trouble with his contemporaries.  Even the disciples were perplexed:

           [Paul's] letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction (2 Pet 3:16).

The point is -- Paul speaks frequently of the "mystery" of the Gospel, hidden before but made manifest now.  Yet there is another mystery, and that is Paul's hermeneutics.  His teaching cannot be derived directly from Old Testament precedent.  Rather, his treatment of Scripture is more reminiscent of Jesus' own teachings:

          "You have heard that it was said....But I tell you...." (Mat 5:21).

Like Jesus, Paul quoted texts that were familiar, even foundational, to Jewish readers, and then turned them upside down.  In this, we get a glimpse of the old Paul, the pre-conversion Saul:  erudite, analytical,  independent, articulate, aggressive.  He was a disputer, strongly opinionated, dogmatic...and passionate about his convictions. After his conversion, he spent years off on his own, somewhere in Arabia and Damascus, studying and praying, but (significantly!) not under the tutelage of the Jerusalem disciples.  He was alone with God and the Scriptures.  He was poring over all the familiar doctrinal ground, revisiting the Scriptures he had used to disprove the Messiahship of Jesus.  Only now, he had the perspective of a reborn spirit and an enlightened mind.  He faced two problems: 1. how to understand the theophany of Jesus Christ on the road to Damascus in terms of the Old Testament revelation, and 2. how to understand the Old Covenant in the light of the Christ Event.  The interplay of these two themes forged both Paul's attitude to Scripture and the content of his Gospel to the Gentiles.