Peter saw a sheet let down from heaven with all kinds of animals on it, including unclean animals,
“When I observed it intently and considered, I saw four-footed animals of the earth, wild beasts, creeping things, and birds of the air. And I heard a voice saying to me, ‘Rise, Peter; kill and eat.’ But I said, ‘Not so, Lord! For nothing common or unclean has at any time entered my mouth’” (Acts 11:6-8).
This was not the first time that Peter had disagreed with the Lord. Perhaps Peter thought this was a test or something, but think of it: the Lord of heaven said to rise, to kill, and to eat, to which Peter said no. He was as inflexible as a rock, and so his name is Peter, a rock, but once the Lord did several other things, Peter finally saw that the distinction between clean and unclean food had disappeared with the nailing of the Law of Moses to the cross, and that he should no longer call Gentiles unclean as well.