1 Cor. 15.56 tells us that the strength of sin is the Law. In Rom. 4.14-15, For if those who depend on the law are heirs, faith means nothing and the promise is worthless, because the law brings wrath. And where there is no law there is no transgression. The purpose of giving the Law some 400 years after the promise was to proliferate sin, The law was brought in so that the trespass might increase. But where sin increased, grace increased all the more (Rom. 5.20 NIV). The Law imprisoned people, Before the coming of this faith,  we were held in custody under the law, locked up until the faith that was to come would be revealed (Gal. 3.23 NIV). The ancient Jews of Israel were unfaithful to the Mosaic regulations just as the Law intended to show them. They could, however, repent and return to the Lord. Many of them did respond to prophetic preaching and hearing of scripture. Given our knowledge of human nature, it is unsurprising that some Jews did not keep kosher:

https://www.timesofisrael.com/those-fishy-reports-on-ancient-israelites-eating-non-kosher-seafood/