Peter's vision is often used as another objection to being able to eat unclean animals. We all know part of the vision. Peter has a dream. God shows a sheet of animals that included unclean animals. God then tells Peter to kill and eat. This is usually where someone stops the story to "prove" that we can eat whatever we want now.
However, there are three major points that are overlooked by taking this stance. The first, Peter tells God "NO!".
Acts 10:14 NIV “Surely not, Lord!” Peter replied. “I have never eaten anything impure or unclean.”
This is Peter, the most zealous of the disciples. If God told Peter something, don't you think he would do it? Something wasn't right about what God was telling Peter. It was something that went against what Peter knew of God's character. Which leads to the second point, and the point of the comic: Peter is confused.
Acts 10:17a NIV While Peter was wondering about the meaning of the vision
We just run with the dream to mean what it literally sounds like: God is telling Peter to eat animals God said not to eat. Peter did not take this interpretation though. He knew some of those animals that were shown were not for food, he would never eat them. If you have ever read the Maccabees, you know how serious those in the faith took not eating those animals, even in the face of death. Peter is showing no different here and knows that this dream isn't a "plain text" reading, but something to be interpreted much like Joseph's dreams. Which leads to the last point, Peter tells us the interpretation, and it's not about food.
Acts 10:28 NIV He said to them: “You are well aware that it is against our law for a Jew to associate with or visit a Gentile. But God has shown me that I should not call anyone impure or unclean."
Peter's vision is all about people. The Jews of the time had a man-made law that forbid them to associate with Gentiles. We even see Peter later going back into following this law later when Paul confronts him in Galatians 2.
Context is important when it comes to forming our Theology. We can't take a verse or two out of context and make it mean what we want it to mean. Even if that interpretation has been around for a very long time.
