Under the Mosaic Law, certain foods were claimed to be unclean such as pork, etc. But union with Jesus Christ frees believers from the Mosaic Law. We are now under a higher law - the law of the filling of the Holy Spirit. Rom. 14:1-14; Rom. 8:2-4.
“Now accept the one who is weak in faith, but not for the purpose of passing judgment on his opinions. One person has faith that he may eat all things, but he who is weak eats vegetables only. The one who eats is not to regard with contempt the one who does not eat, and the one who does not eat is not to judge the one who eats, for God has accepted him. Who are you to judge the servant of another? To his own master he stands or falls; and he will stand, for the Lord is able to make him stand.” (Romans 14:1-4, NASB)
“I know and am convinced in the Lord Jesus that nothing is unclean in itself; but to him who thinks anything to be unclean, to him it is unclean.” (Romans 14:14, NASB)
“For the law of the Spirit of life in Christ Jesus has set you free from the law of sin and of death. For what the Law could not do, weak as it was through the flesh, God did: sending His own Son in the likeness of sinful flesh and as an offering for sin, He condemned sin in the flesh, so that the requirement of the Law might be fulfilled in us, who do not walk according to the flesh but according to the Spirit.” (Romans 8:2-4, NASB)
Under this new law, food categories of clean and unclean are gone. All foods are clean. These are only clean to the believer who is oriented to grace and understands the pertinent doctrine. The Mosaic Law rejected certain meats and Jewish believers were still offended when others ate pork. The Gentile’s prejudice was based on eating meat offered to idols.
The legalistic believer is weak and lives by their background and prejudices from human viewpoint. They are minus doctrine and because of this, they apply their personal prejudices to every situation and tries to superimpose their taboos on others. It obscures life for themselves and others. The Christian way of life is what you should think. Because of all these factors, the legalistic believer takes a clean thing (food) and makes it unclean by human viewpoint. See category on Laws for Doubtful Things.