I have never eaten ... - In the Old Testament God had made a distinction between clean and unclean animals. See Leviticus 11:2-27; Deuteronomy 14:3-20. This law remained in the Scriptures, and Peter pled that he had never violated it, implying that he could not now violate it, as it was a law of God, and that, as it was unrepealed, he did not dare to act in a different manner from what it required. Between that law and the command which he now received in the vision there was an apparent variation, and Peter naturally referred to the well known and admitted written Law. One design of the vision was to show him that that Law was now to pass away.
That is common - This word properly denotes “what pertains to all,” but among the Jews, who were bound by special laws, and who were prohibited from many things that were freely indulged in by other nations, the word “common” came to be opposed to the word “sacred,” and to denote what was in common use among the pagans, hence, that which was “profane,” or “polluted.” Here it means the same as “profane,” or “forbidden.”
Unclean - Ceremonially unclean; that is, what is forbidden by the ceremonial law of Moses.