Richard Hays has written a very significant book on Paul’s use of Scripture. In it he argus that Paul will often allude to the OT without formally quoting it.
CH Dodd proposed something similar when he spoke of how when a NT writer does make a formal quotation of the OT they often had the total context of the OT passage in view. However, Hays goes further and argues that we cannot adequately understand Paul ‘unless we seek to situate his discourse appropriately with what Hollander calls the “cave of resonant signification” that enveloped him: Scripture’. That is unless you read Paul as someone who was saturated in the OT, you will never understand him.
To take a couple of examples, when Paul says in Romans 2:6 that God will ‘render to every man according to his works’ he does not (despite the punctuation of most English versions) actually signal that he is quoting from the OT, but his words are lifted exactly from Ps 61:13 or Proverbs 24:12. That is an easy example as he is referring to a whole phrase.
Sometimes, however, he will allude to an OT passage with just a word or part of a phrase.
To take an example. In Phil. 1:19 Paul says:
for I know that through your prayers and the help of the Spirit of Jesus Christ this will turn out for my deliverance
Very few commentators pick up on the fact that the last part of the statement is an almost verbatim quotation of the Septuagint (the Greek translation of the OT) of Job 13:16. This raises two very important questions:
i. How do we know if Paul is actually alluding deliberately to an OT passage?
ii. If he is alluding to it, how much exegetical weight should we give this fact?