In Acts 10, why does Peter not already know that he can eat foods previously regarded as 'unclean'?
Because Peter was aware that Jesus did not nullify the food laws of Leviticus 11.
In Mark 7, Jesus rebukes the Pharisees and scribes for condemning those who eat with "unwashed" hands. Jesus does not rebuke any of the dietary laws, but only the addition of requiring "washed hands" before eating. This kind of addition was actually prohibited by Moses:
“Now, O Israel, listen to the statutes and the judgments which I teach you to observe, that you may live, and go in and possess the land which the Lord God of your fathers is giving you. 2 You shall not add to the word which I command you, nor take from it, that you may keep the commandments of the Lord your God which I command you." -Deuteronomy 4:1-2 (NKJV)
While God did command that certain things should not be eaten in Leviticus 11 (such as pigs, camels, rabbits, etc.), there was never a command that you must wash your hands in a special way before eating:
1 Then the Pharisees and some of the scribes came together to Him, having come from Jerusalem. 2 Now when they saw some of His disciples eat bread with defiled, that is, with unwashed hands, they found fault. 3 For the Pharisees and all the Jews do not eat unless they wash their hands in a special way, holding the tradition of the elders. 4 When they come from the marketplace, they do not eat unless they wash. And there are many other things which they have received and hold, like the washing of cups, pitchers, copper vessels, and couches. -Mark 7:1-4 (NKJV)
Jesus corrected the false view that not washing your hands before eating defiled you. This is made explicit in Matthew's gospel:
17 Do you not yet understand that whatever enters the mouth goes into the stomach and is eliminated? 18 But those things which proceed out of the mouth come from the heart, and they defile a man. 19 For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies. 20 These are the things which defile a man, but to eat with unwashed hands does not defile a man.” -Matthew 15:17-20 (NKJV)
In fact, it was Peter himself who was the one to ask Jesus for clarification about this incident:
Then Peter answered and said to Him, “Explain this parable to us.” -Matthew 15:15
Also note that in both Matthew 15 and Mark 7 the disciples are only mentioned eating bread, so this is even more evidence that Jesus was not referring to laws such as those found in Leviticus 11, which concern different kinds of flesh foods:
1 Then the scribes and Pharisees who were from Jerusalem came to Jesus, saying, 2 “Why do Your disciples transgress the tradition of the elders? For they do not wash their hands when they eat bread.” -Matthew 15:1-2 (NKJV)
1 Then the Pharisees and some of the scribes came together to Him, having come from Jerusalem. 2 Now when they saw some of His disciples eat bread with defiled, that is, with unwashed hands, they found fault. -Mark 7:1-2 (NKJV)
Unfortunately Bob Utley is mistaken, for even in Revelation God still makes a distinction between the 'clean' and the 'unclean' when one of His angels cries out against Babylon:
After these things I saw another angel coming down from heaven, having great authority, and the earth was illuminated with his glory. 2 And he cried mightily with a loud voice, saying, “Babylon the great is fallen, is fallen, and has become a dwelling place of demons, a prison for every foul spirit, and a cage for every unclean and hated bird! -Revelation 18:1-2 (NKJV)
While Peter did initially wonder about the meaning of his vision in Acts 10,
Now while Peter wondered within himself what this vision which he had seen meant, behold, the men who had been sent from Cornelius had made inquiry for Simon’s house, and stood before the gate. -Acts 10:17 (NKJV)
he realizes and explains the true meaning shortly thereafter:
Then he [Peter] said to them [Cornelius & friends], “You know how unlawful it is for a Jewish man to keep company with or go to one of another nation. But God has shown me that I should not call any man common or unclean. -Acts 10:28
Jesus did not nullify the food laws of Leviticus 11. Peter knew this, and is why he initially wondered over the meaning of his vision in Acts 10. But he soon realizes the true meaning of the vision: that it was not about animals or food at all, but instead that he should call no man common or unclean.
A comment asked about Mark 7:19, where Jesus declares all foods clean. I have quoted from several versions and marked verse 19 in bold below.
NKJV17 When He had entered a house away from the crowd, His disciples asked Him concerning the parable. 18 So He said to them, “Are you thus without understanding also? Do you not perceive that whatever enters a man from outside cannot defile him, 19 because it does not enter his heart but his stomach, and is eliminated, thus purifying all foods?”
NIV17 After he had left the crowd and entered the house, his disciples asked him about this parable. 18 “Are you so dull?” he asked. “Don’t you see that nothing that enters a person from the outside can defile them? 19 For it doesn’t go into their heart but into their stomach, and then out of the body.” (In saying this, Jesus declared all foods clean.)
ESV17 And when he had entered the house and left the people, his disciples asked him about the parable. 18 And he said to them, “Then are you also without understanding? Do you not see that whatever goes into a person from outside cannot defile him, 19 since it enters not his heart but his stomach, and is expelled?” (Thus he declared all foods clean.)
My response is specific to the rendering of "he declared all foods clean." As mentioned in the main section of my answer, this conflict was over unwashed hands and that the disciples were only eating bread to begin with. The phrase "Thus he declared all foods [βρωματα] clean" uses the word "βρωματα" for "foods."
Thayer's definition for βρωματα (βρωματα is the plural of βρωμα):
- that which is eaten, food
So the last clause of verse 19 in the NIV/ESV is saying: _"Thus he declared all that which is eaten clean" as in, all that which is eaten can be eaten without having to wash your hands in a special way.
βρωματα does not exclude flesh foods, with the context of a situation indicating whether or not flesh foods would be thought of when the word is used. But my point is twofold:
- the disciples were only eating bread so flesh foods were not even being considered, and
- even if flesh foods were being considered, "unclean" flesh foods would not have been since "unclean" flesh foods were not to be eaten to begin with. Thus βρωματα, that which is eaten, would definitely exclude all "unclean" flesh foods in Mark 7:19.