Was Peter mixing law and grace when he continued to practice the law in Acts 10?
No. Peter was not mixing law and grace; he was still practicing the law because he had not yet been told to stop temple worship or that he was no longer under the law. As a Jewish believer, he had not yet become aware of the doctrines of grace, so he remained much aware of the law.
Les teaches that Peter’s refusal to eat was not a mixing of law and grace. Peter says, “Not so, Lord, for I have never eaten anything that is common or unclean,” because he is under the law. God had explicitly defined clean and unclean animals, fowl, and fish in Leviticus. Les makes the point that there is a vast chasm between law and grace, and you cannot mix law and grace in the church age. But Peter is still practicing the law. Is he mixing it? No. Peter has not been told to stop temple worship, nor has he been told he is no longer under the law. He is under grace. Peter is like the followers of Christ in His earthly ministry: a Jew who believed Jesus was the Christ, had repented and been baptized. He is the typical Jewish believer, but he is not yet aware of the doctrines of grace, so he remains much aware of the law. The second time the voice says, “What God hath cleansed, call that not common.”