The Jews, and especially the Pharisees, considered that their rabbincal interpretation of the Law of Moses were as binding as the actual written text. For example:
Gal 2:11 Now when Peter had come to Antioch, I withstood him to his face, because he was to be blamed;
Gal 2:12 for before certain men came from James, he would eat with the Gentiles; but when they came, he withdrew and separated himself, fearing those who were of the circumcision.
Gal 2:13 And the rest of the Jews also played the hypocrite with him, so that even Barnabas was carried away with their hypocrisy.
Gal 2:14 But when I saw that they were not straightforward about the truth of the gospel, I said to Peter before them all, "If you, being a Jew, live in the manner of Gentiles and not as the Jews, why do you compel Gentiles to live as Jews?
Gal 2:15 We who are Jews by nature, and not sinners of the Gentiles,
Gal 2:16 knowing that a man is not justified by the works of the law but by faith in Jesus Christ, even we have believed in Christ Jesus, that we might be justified by faith in Christ and not by the works of the law; for by the works of the law no flesh shall be justified.
Now why this is germane is that there was NO law in scripture, from the Lord, that said that Jews couldn't dine with gentiles. But over the years the Jews had lost their way and had put up walls and barriers that not only separated them from gentiles, but also separated gentiles from the gospel. This isn't what God intended.
Eph 2:11 Therefore remember that you, once Gentiles in the flesh—who are called Uncircumcision by what is called the Circumcision made in the flesh by hands—
Eph 2:12 that at that time you were without Christ, being aliens from the commonwealth of Israel and strangers from the covenants of promise, having no hope and without God in the world.
Eph 2:13 But now in Christ Jesus you who once were far off have been brought near by the blood of Christ.
Eph 2:14 For He Himself is our peace, who has made both one, and has broken down the middle wall of separation,
Eph 2:15 having abolished in His flesh the enmity, that is, the law of commandments contained in ordinances, so as to create in Himself one new man from the two, thus making peace,
These ordinances were the countless rules and regulations developed by the jewish religion over the centuries...the burdens that even they couldn't bear.
Eph 2:16 and that He might reconcile them both to God in one body through the cross, thereby putting to death the enmity.
This is what the Lord intended all the time...a unfied people unified by obeying and submitting to the Lord and his commands.
It took time for Peter and some other Jews to learn and understand the division between what was scriptural and what was cultural and traditional.