To suggest that one cannot find "doctrine" from parts of the Bible not written to believers in Messiah 2,000 years later is like suggesting that a person cannot learn about the love between his mother and father by reading a letter composed to the mother from the father. Indeed the letter is written from one to the other, not to the world in general, but much of that love between them can be learned, (that's doctrine) by way of reading those words written to someone else.
To suggest that no doctrine can come from parts of the Bible which were not written to us today is in the same vein. Of course promices given to a particular people were not given to mankind in general, judgements on particular nations are not to be spread to mankind in general, yet much doctrine can be learned regarding the nature of our God and His ways. To limit ones doctrine to the writtings of but one of Scriptures inspired writters is to miss the universal truths which are contained in each and every chapter of all Scripture.
To suggest that no doctrine can come from parts of the Bible which were not written to us today is in the same vein. Of course promices given to a particular people were not given to mankind in general, judgements on particular nations are not to be spread to mankind in general, yet much doctrine can be learned regarding the nature of our God and His ways. To limit ones doctrine to the writtings of but one of Scriptures inspired writters is to miss the universal truths which are contained in each and every chapter of all Scripture.