Paul (Saul) was a Jew who was well on his way up in Judaism (Acts 9:1-2, 26:4-5, 10-11 Gal 1:13-14, Phil 3:4-6). So he knew his scripture (O.T.) and as he writes his epistles, the Holy Spirit brings these things to mind and reveals the hidden truths for the believers in Christ. Sometimes he quotes directly and other times allude to the ideas. He wrote to Titus.
2:5 To be discreet, chaste, keepers at home, good, obedient to their own husbands, that the word of God be not blasphemed.
Genesis 17:15 And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be.
Genesis 18:6 And Abraham hastened into the tent unto Sarah, and said, Make ready quickly three measures of fine meal, knead it, and make cakes upon the hearth. Sarah the example
2:14 Who gave himself for us, that he might redeem us from all iniquity, and purify unto himself a peculiar people, zealous of good works.
Exodus 19:5 Now therefore, if ye will obey my voice indeed, and keep my covenant, then ye shall be a peculiar treasure unto me above all people: for all the earth is mine:
Deuteronomy 7:6 For thou art an holy people unto the LORD thy God: the LORD thy God hath chosen thee to be a special people unto himself, above all people that are upon the face of the earth. Believers are peculiar people.
3:8 This is a faithful saying, and these things I will that thou affirm constantly, that they which have believed in God might be careful to maintain good works. These things are good and profitable unto men