The Progressive Content of Paul's Epistles After Acts 28:28, when Paul turns his focus wholly to the Gentiles, one sees a change in the content of his epistles. With careful study one finds much less references to Old Testament quotes and to Jewish customs and traditions. This post will provide some insights as to why.

Interesting Fact: In Paul's writings from after Acts 28, —that is in his "Prison Epistles - Ephesians to Colossians", —one finds minimal references to Old Testament quotes or any mention of Jewish customs or traditions. Why is this? My answer would be that in Acts 28:28 he mentions that his attention will now turn to the Gentiles as they were receptive to his message. Paul spent years in his early ministry trying to bring the Jews to salvation through the grace gospel, but they would not hear. The fact that there is little in the subsequent prison epistles regarding the Jewish prophetic program and their customs validates for me that his heart was now set solely on a 'pure' grace message alone.

After Acts 28, Paul went on trial and was regularly under house arrest. It is at this point that he turned his focus exclusively towards the Gentiles as he became convinced that the Jews were hard of hearing and heart and would not believe. Before this event, Paul would typically enter a town or city and go to the synagogues first. During his missionary journeys he was always still convinced that he could turn his countrymen to believe in the Lord (Rom.10:1; Rom.9:1-5). His early epistles, (1 & 2 Thessalonians, Galatians, 1 & 2 Corinthians and Romans), had references to Jewish customs and traditions and also quotes from the Old Testament, specifically targeted to his Jewish audience regarding the Jewish prophetic program and to prove the identity of Jesus Christ. Also, below the Old Testament quotes are many more scriptures from these books that I've extracted that show references in his writings by specific category headings. Please scan through them and see my comments that I hope will provide some answers to your question.

Let's first look at some Old Testament quotes. 

There are a few minor entries in Ephesians, Philippians and Colossians that mention some text from the Old Testament. However, these quotes are used in a general fashion and are relevant to both Jew and Gentile in the churches Paul established. In other words, the references below are not specific to the Jewish audience only in these Gentile churches, typically in the attempt to convince the Jews to the identity of their Messiah. See the following:

Pual wrote:

• Ephesians 4:8 Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men.

Parallel quote for the Old Testament:

• Psalms 68:18 Thou hast ascended on high, thou hast led captivity captive: thou hast received gifts for men; yea, for the rebellious also, that the LORD God might dwell among them.

Paul wrote the following that basically references one of the Mosaic Laws. This is a general statement that both Jew and Gentile can relate to.

• Ephesians 6:1 Children, obey your parents in the Lord: for this is right. 2 Honour thy father and mother; (which is the first commandment with promise;)

I'm not sure if you have other passages in mind that you can think of, but I scanned again through Paul's prison epistles to find anything more from the Old Testament and didn't find anything more of significance.

Other Significant Quotes

With regards to other things Paul wrote in these prison epistles (Ephesians, Philippians and Colossians) that relate directly to Jewish context, the only other entry is the following. Paul wrote about his heritage as a Jew, but you will notice that he was teaching a specific point here to both Jews and Gentiles and ended it abruptly by saying that his Jewish heritage this means absolutely nothing and he counts these as loss for the sake of Christ. Paul was therefore not trying to convince any Jews in his audience or trying to promote the preeminence of the Jews or the Jewish prophetic program.

• Philippians 3:4 Though I might also have confidence in the flesh. If any other man thinketh that he hath whereof he might trust in the flesh, I more: 5 Circumcised the eighth day, of the stock of Israel, of the tribe of Benjamin, an Hebrew of the Hebrews; as touching the law, a Pharisee; 6 Concerning zeal, persecuting the church; touching the righteousness which is in the law, blameless. 7 But what things were gain to me, those I counted loss for Christ.

Jewish References prevalent in his Early Epistles

The rest of the scriptures below are found only in his early writings and have clear reference to Jewish customs and traditions. These below would make no sense to Gentile converts in his day as the Gentiles would be unfamiliar with these customs and patterns. These are only found in his early epistles from Thessalonians to Galatians. None of these things are even hinted to in his prison epistles (since his focus was not in convincing Jews anymore, but rather to speak a pure grace doctrine applicable to the church, the Body of Christ.)

Head Covering

• 1 Corinthians 11:4 Every man praying or prophesying, having his head covered, dishonoureth his head. 5 But every woman that prayeth or prophesieth with her head uncovered dishonoureth her head: for that is even all one as if she were shaven. 6 For if the woman be not covered, let her also be shorn: but if it be a shame for a woman to be shorn or shaven, let her be covered. 7 For a man indeed ought not to cover his head, forasmuch as he is the image and glory of God: but the woman is the glory of the man.

Hairstyles

• 1 Corinthians 11:14 Doth not even nature itself teach you, that, if a man have long hair, it is a shame unto him? 15 But if a woman have long hair, it is a glory to her: for her hair is given her for a covering.

Reference to observances of Jewish Law

• Romans 2:23 Thou that makest thy boast of the law, through breaking the law dishonourest thou God? 24 For the name of God is blasphemed among the Gentiles through you, as it is written.25 For circumcision verily profiteth, if thou keep the law: but if thou be a breaker of the law, thy circumcision is made uncircumcision. 26 Therefore if the uncircumcision keep the righteousness of the law, shall not his uncircumcision be counted for circumcision? 27 And shall not uncircumcision which is by nature, if it fulfil the law, judge thee, who by the letter and circumcision dost transgress the law? 28 For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: 29 But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.
  
• Galatians 3:21 Is the law then against the promises of God? God forbid: for if there had been a law given which could have given life, verily righteousness should have been by the law. 22 But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe. 23 But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed. 24 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith.

Covenants

• Romans 9:4 Who are Israelites; to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and the promises;
  
• Galatians 3:17 And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.

Using Jewish Historical References

• 1 Corinthians 10:1 Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea; 2 And were all baptized unto Moses in the cloud and in the sea; 3 And did all eat the same spiritual meat; 4 And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ.

Jewish Traditions

• 2 Thessalonians 2:15 Therefore, brethren, stand fast, and hold the traditions which ye have been taught, whether by word, or our epistle.
  
• 2 Thessalonians 3:6 Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us.

Feasts

• 1 Corinthians 5:7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us: 8 Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.

Personal Comments

Please note that through my personal views and discovery, I have settled on the details above as a valid understanding of the progression of Paul's writings from when he was targeting Jews specifically, all the way to when he recognized the futility of convincing the Jews and started to focus more purely on the mysteries and grace doctrine in his writings from prison. 

Also, as quoted above, I'm not at all saying that Thessalonians to Galatians contains 'less pure' grace doctrine!! Not at all!! It simply means that one has to read these epistles more carefully and recognize by context when Paul is specifically targeting Jewish converts. For example, hair length and head coverings, law, and covenants are certainly not relevant to us under grace.