In Acts 10, the apostle Peter is up on a roof praying and has a vision in which he is confronted by a voice from heaven. A sheet is lowered down from heaven which contained all kinds of animals that God had said in the Old Testament were unclean for Jews to eat. Then this voice from heaven tells Peter to take them and eat them. Peter refuses and says that he has never eaten anything unclean. The voice speaks a second time and says: “Do not call anything impure that God has made clean” (Acts 10:15). The Greek word translated "made clean" is in the aorist form, that is in the past tense. So the voice is explicitly saying that at some time in the past God made all these animals clean.
But when did God make all these things clean? It most certainly was not anywhere in the Old Testament. Neither was it at the time that the sheet was being lowered down. If so, it would make no sense for Peter to refuse to eat the animals in the sheet and say: “I have never eaten anything impure or unclean” (Acts 10:14). The voice from heaven speaks as if Peter should know that these animals had been made clean, so why does Peter respond as if he does not know?
It had to have been something previous in Peter's experience that he had not yet fully processed.
This could be one of those unanswered mysteries, in which we just don't have enough information to answer that question. But in chapter 7 of the Gospel of Mark, there is an editorial comment inserted after Jesus' teaching about his disciples not washing their hands when they eat. Jesus' teaching on the matter is summed up in Mark 7:15: "Nothing outside a person can defile them by going into them. Rather, it is what comes out of a person that defiles them."
Then, when the disciples are alone with Jesus, they ask him to explain. He responds to them in Mark 7:18-19: "Are you so dull?” he asked. “Don’t you see that nothing that enters a person from the outside can defile them? For it doesn’t go into their heart but into their stomach, and then out of the body.” Immediately following these words, we have this editorial comment from Mark: "(In saying this, Jesus declared all foods clean.)"
The words quoted above are the New International Version's attempt to translate the Greek. But the word "declared" is not in the Greek. The only verb here is the same one translated "made clean" in Acts 10:15: καθαρίζω, katharizō, "make clean." Literally, this editorial comment in Mark 7:19 reads: "...making all things clean."
When did God make all things clean? He did it when Jesus spoke these words recorded in Mark 7. But he did not do so explicitly and Peter had not yet fully processed Jesus' words, bringing them to their logical conclusion. He does not do so until after confronted by the voice from heaven in Acts 10.
This is just one more interesting little "undesigned coincidence" that confirms that the Biblical writers are writing down things that actually happened and not simply manufacturing stories on the fly.
But wait, there's more! Curiously, though Jesus' teaching about washing hands is recorded in Matthew 15, only Mark records this editorial comment that Jesus was making all foods clean. Why is that interesting? Because Mark is recording the testimony of Peter and it is Peter who is confronted by the voice from heaven. Evidently, this confrontation with the heavenly voice brought Jesus' words about washing hands to mind and it all made sense. And so, this editorial comment reflects the impact of that interaction in Peter's experience. And by the way, guess who asks Jesus to explain the parable. Yep, Peter (Matthew 15:15). And he does it at Peter's house (see Howson's remarks below).
So this not only becomes another small piece of evidence in our overwhelming cumulative case for the historicity of the Gospels but also for the traditional authorship of the Gospel of Mark as passed on to us by the apostolic father Papias (that Mark is recording Peter's testimony).
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Below is the full text of John Saul Howson's remarks on the this undesigned coincidence in his book Horæ Petrinæ: or, Studies in the Life of St. Peter (London: 1883), p.35:
Among the sayings uttered by our Lord which may be truly termed parables, though not always formally classed under that designation, was this: "There is nothing from without a man, that entering into him can defile him: but the things which come out of him, those are they that defile the man."1 The first part of this sentence, though only referring to outward things, carries with it large and far-reaching spiritual meanings. Hence it may correctly be termed a parable.
These words, too, like those with which we were occupied in the preceding chapter, were spoken at Capernaum. The Lord had come back from across the lake; and certain Pharisees, with some scribes from Jerusalem, visited Him, and found fault with His disciples for eating with unwashen hands. This led to a stern rebuke of formality and hypocrisy, and of that religion which consists of an observance of externals. And now the Lord, having finished with the Pharisees and scribes, "called all the people," and spoke the words above quoted, with one emphatic addition which made them more memorable: "Hearken unto Me every one of you, and understand: If any man have ears to hear, let him hear."'
1 Mark vii. 15.
Now we come to the point of special connection with St. Peter. In order to see it distinctly, we must put together the accounts of St. Matthew and St. Mark. Both these evangelists tell us that the disciples afterwards privately asked the meaning of the parable. But one of them informs us of the place where this conversation occurred; the other tells us who asked the question that led to Christ's answer. St. Mark says that it was "when He was entered into the house from the people" that this private conversation took place. That was the house of Simon and Andrew. But St. Matthew tells us of something still more definite and personal: "Peter said unto Him, Declare unto us this parable." Peter is, as usual, ready with his words; and while, doubtless, honestly eager for instruction, by no means retires behind the others into the background. We have to thank his impetuosity, if we may reverently say so, for a very important result. For, after our Lord has explained that what goes into a man's mouth merely follows physical laws, and has no necessary effect on his character, He declares to us the solemn truth that the evil thoughts and foul desires which come from within do defile him morally and spiritually. After this statement, St. Mark adds, according to the true reading of the manuscripts: "This He said, cleansing all meats," pronouncing all food pure. 1 Matt. xv. 1-20; Mark vii. 1-23.
This is not the place for any remarks in detail on the critical question here involved. But the fact of the true reading, which has been very curiously preserved, may be stated very confidently.1 And from this fact the mind passes, by a rapid process of thought, to another occasion on which St. Peter was afterwards concerned with the same subjects. Can we doubt that a remembrance of his Lord's words came into St. Peter's mind in connection with the case of Cornelius, if not with the flash of a sudden conviction, yet with a gradual, and, in the end, irresistible persuasion, during the vision at Joppa, or in conversation at Caesarea? The Lord had spoken the words in answer to a pointed question; and the question had been asked in Peter's house and by Peter himself. We should note, too, that the identical Greek word for "cleansing," or "declaring pure," is used in the two cases. It is a most expressive word, which, besides teaching the lesson for the moment of the worthlessness of all merely external observances, proclaims all caste to belong to temporary states of society, and to be out of harmony with the true spirit of the Gospel. This link of connection between Capernaum and Caesarea, between a parable, at first obscure, and the broad, luminous teaching of the universality of the religion of Christ, is full of interest, and has not been observed so carefully as it deserves.
