9 The next day, as they were on their journey and approaching the city, Peter went up on the housetop about the sixth hour  to pray. 10 And he became hungry and wanted something to eat, but while they were preparing it, he fell into a trance 11 and saw the heavens opened and something like a great sheet descending, being let down by its four corners upon the earth. 12 In it were all kinds of animals and reptiles and birds of the air. 13 And there came a voice to him: “Rise, Peter; kill and eat.” 14 But Peter said, “By no means, Lord; for I have never eaten anything that is common or unclean.” 15 And the voice came to him again a second time, “What God has made clean, do not call common.” 16 This happened three times, and the thing was taken up at once to heaven.
17 Now while Peter was inwardly perplexed as to what the vision that he had seen might mean, behold, the men who were sent by Cornelius, having made inquiry for Simon's house, stood at the gate 18 and called out to ask whether Simon who was called Peter was lodging there. 19 And while Peter was pondering the vision, the Spirit said to him, “Behold, three men are looking for you. 20 Rise and go down and accompany them without hesitation, for I have sent them.” 21 And Peter went down to the men and said, “I am the one you are looking for. What is the reason for your coming?” 22 And they said, “Cornelius, a centurion, an upright and God-fearing man, who is well spoken of by the whole Jewish nation, was directed by a holy angel to send for you to come to his house and to hear what you have to say.” 23 So he invited them in to be his guests. (Acts 10:9-23 (ESV)
God declares all food clean. Peter points this out in Mark 7 too. That is Peter’s gospel. But he uses that episode to show how Jesus declared all food to be clean. You get the impression from Mark that that was the subject of not a few sermons Peter delivered on that event. But Peter himself didn’t really understand it that way at the time. This understanding, the fullness of what was said and done comes later as Peter remembers it after his vision at Simon the Tanner’s house.
The vision is part of Peter’s preparation for what is about to happen. It is part of the preparation for the mission to the gentiles, and it occurs as Peter is praying before a meal, ostensibly to sanctify the meal as everything is sanctified through prayer and the word. Prayer is a regular part of Peter’s life, and one that he does even more than at the common set times for prayer. The sixth hour was not a common time for people to be praying. But at least at this time, as Peter is waiting for some food to be prepared he prays. And during the prayer falls into a trance, and extasy. It wasn’t something he pursued, but it was something given to him, and it was given for the purpose of revelation.
This is a bit different from modern day mystics who try every sort of means to induce these trances and to feel at one with God, or the universe or what have you. Or expect that this sort of an experience is something everyone is entitled to. If you want to induce a trance, or an ecstatic moment, well then you will. If for instance you expect the “burning of the Bosom” to occur when you read the book of Mormon, it will likely occur for you. This is the same sort of thing that is often induced at Pentecostal rallies, and conventions of whirling dervishes. And that is the problem. If the experience is self-induced, and expected, well it is manufactured. The experience is real, don’t get me wrong, but the truth of it, or what it means is anybody’s guess.
Peter himself is horrified by the implications of the vision. I imagine the prospect of him eating pork has about the same effect as me when I contemplate Gefilte Fish. The stuff just looks gross. Food is at the bedrock of culture. And somethings we grow up eating, and think nothing of, but others look at it as disgusting. All this to say, this not only is a vision Peter didn’t pursue, it is a vision he didn’t want. He got it any way. And it was meant to convince him. It’s a rather strange thing there, then, the relation of this to Mark 7.
Acts actually has an awkward place in the canon of scripture. The Early church didn’t preach much from it. It had apostolic sanction and people knew that there could be enlightening truth in it, but they were hesitant to use it in the creation of dogma, because it really records a lot of things that happen, but doesn’t really set a whole lot down dogmatically. I mean, one can figure that what Peter proclaims in his sermon on Pentecost has dogmatic implications for all time. But trying to set down that because something happened one way once, now it has to happen that way for all time, that is another proposition altogether.
It seems Peter himself is a little reluctant to tall people, I had a vision and now all foods are clean. The vision itself convinces him and prepares him to go to the gentiles. But when he is going to convince others, well then he returns to the words of Christ that others have witnessed. He may also mention his vision, but he preaches from the words of Christ.