A while ago, someone commented on this passage from Luke 17:20-21 – 
Now when He was asked by the Pharisees when the kingdom of God would come, He answered them and said, “The kingdom of God does not come with observation; nor will they say, ‘See here!’ or ‘See there!’ For indeed, the kingdom of God is within [ἐντός/entos] you.” (I quote the NKJV here, since that resembles the Spanish version that the person was using)
The man went on to comment that, the early church invented this saying of Jesus, decades after his earthly ministry, to explain why Son of Man had not returned as anticipated; that it reflects a church that is trying to accommodate itself to a prolonged stay in this world; that the verse is one-of-a-kind, since Jesus never spoke of God’s kingdom as having come in the present time [sic]. To paraphrase his interpretation: “Luke 17:21 describes the kingdom as an inner, mystical, not apocalyptic experience; it is inside the human heart! And Jesus never said uttered that saying, and nowhere else did he say that the kingdom had come.”
MY RESPONSE, using the same sort of categories that he had used:
The question of whether the authors of the New Testament were trying to cope with the delay of parousia is indeed an important question, best synthesized by Erich Grässer in 1957 in his Das Problem der Parusieverzögerung in den synoptischen Evangelien. But let’s take all aspects of this text into account.
I put to you that, first, this is not the only place in the gospels where the kingdom of God “has come.” Those who regard Q as a source for Matthew and Luke, claim (in The Critical Edition of Q in the Hermeneia series, for example, which also classifies 17:20-21 as early, Q, material) that Q also contained this dominical saying, “But if it is by the Spirit/finger of God that I cast out the demons, then the kingdom of God has come to you.” (Matthew 12:28, Luke 11:20). That is, the kingdom is not only a place, but is also present when and wherever God acts with real power. The fact that God acts as king and strips away Satan’s kingdom in Jesus’ exorcisms means that God’s kingship is dynamically present. The Lucan version includes the use of the aorist ἐφθάσεν/ephthasen, with a perfective aorist of “has arrived.” Jesus’ claim that the kingdom has come “upon you” in 11:20 thus parallels “among you” in 17:21, in my view. The rabbis, for their part, were focused on Daniel 9 and their calculations of the time of the kingdom’s coming. Jesus commands that they open their eyes and look at the acts of God right in their midst, specifically in the person of God’s Son.
Second, you say, “Jesus never mentioned to the Israelites that the kingdom would be implanted in hearts.” This is technically correct, but not the whole picture: the later rabbis understood the “kingdom of God” as the yoke that the Jew takes upon himself, the obligation to obey the Torah. While this may help us better understand the yoke metaphor in Matthew 11:28-30 (“take my yoke upon you”), it may not be relevant to this text, since Jesus does not say that the kingdom, as you suggest, is “in people’s hearts.” Why? A. It doesn’t mention people’s hearts! B. It uses the preposition ἐντός/entos. This preposition can mean “within” a space or object but often means “between, among” when used with a plural object. As the BDAG lexicon says of entos in Luke 17:21, “ἐ. ὑμῶν is probably patterned after ἐν σοί (=[God] is among you) Is 45:14, but with Lk preferring ἐντός in the sense among you, in your midst, either now or suddenly in the near future.” Isaiah does not speak of God dwelling in the individual, but of being present among the members of the nation. English versions heavily favor that meaning: “in your midst” (e.g., ESV, NET, NASB, NIV); “among” (ISV, BDAG, NEB, NJV, NLT, NRSV). BDAG adds that “within you, in your hearts” is a possibility, but then dismisses it as its meaning in Luke 17. Note that the object of the preposition ἐντός/entos is not the followers of Jesus, but his enemies. According to your interpretation, Jesus would be saying, “The kingdom of God is already in your hearts, even though you refuse me.” It is a very unlikely meaning, which I cannot imagine a redactor of Luke inventing. D. The idea that the kingdom of God is interior, mystical, spiritual: historically, this was perhaps the favorite interpretation, but most scholars today seem to reject it. In recent times, the inner, mystical interpretation was promoted by nineteenth-century Protestants Wilhelm Herrmann and notably by Adolf von Harnack in his foundational work, What is Christianity? (1900, available at CCEL). He considered it “a still and mighty power in the hearts of men.” In the early 20th century, Weiss, Schweitzer, Barth, and others overturned this trend.
Third, BDAG claims that Luke 17:21 has sparked much “debate,” which is an understatement! I think it’s the most difficult of Jesus’ kingdom sayings. The fact that it is “debated” means that your interpretation (“the kingdom of God is in people’s hearts”), while a possibility, is a remote one. Therefore, your assumption that, “It is well known that the Jesus of history, who was waiting for a future and imminent kingdom of God, never mentioned to the Israelites that the kingdom would be implanted in hearts,” while correct, assumes a priori that Jesus does indeed say that “the kingdom would be implanted in hearts” here in Luke. And that is disputed, and debatable!
Fourth, it would have to be assumed that Luke’s editor is very clumsy, since he follows v. 21 with an apocalyptic discourse, apparently oblivious to contradiction! In 17:22-24 – “Then he said to the disciples, “The days are coming when you will long to see one of the days of the Son of Man, and you will not see it. They will say to you, ‘Look there!’ or ‘Look here!’ Do not go, do not set off in pursuit. For as the lightning flashes and lights up the sky from one side to the other, so will the Son of Man be in his day.” This is not to mention Luke’s other “apocalyptic” texts.
Fifth, those who assume that Luke used Q, label 17:21 as an apothegm of that tradition. That is, even for those who credit only a highly circumscribed set of Jesus-sayings, would make the text one of Jesus’ earliest sayings, predating Paul’s letters, for example. Too early, perhaps, to imagine that the church wants to accommodate itself to the world.
Sixth, Paul, whose epistles predate Luke’s gospel, already speaks of the kingdom as a present reality, in the mid-50s, and in my opinion, in his oral teaching at least since AD 49. It affirms that since God has intervened in its creation through the Spirit, the kingdom of God currently shapes our ethics. Both Rom 14:17 and 1 Cor 4:20 use the framework “the kingdom of God is not….”, which implies a norm in the present tense by which people can measure their behavior.
Your interpretation is that Luke’s editor put these words in Jesus’ mouth when the church had established itself in the world. I guess you’re referring to the last quarter of the first century, when the church was persecuted more by the synagogue than by the empire. And you claim that 17:21 was a means of accommodating the church to the world, though it means something totally different: “The kingdom of God is IN the heart of EVERY ONE OF YOU, the Pharisees! “
That is why I suggest that v. 21 be taken, as affirmed even by many minimalist scholars, as an authentic saying of Jesus; that the author or editor saw no contradiction between 17:20-21 and the apocalypse of 17:22-37. Given the above conserations, I would paraphrase it: “Do not concentrate on the future kingdom, when you can already see clearly in my actions that God is acting with the power of the kingdom among you: in the preaching of the good news, in exorcisms, in healings.”
To add a practical thought for this blog post: The application for today’s church might be too obvious to mention: we live in a day where Christians spend millions on rapture lit, listen to date-setters, argue the exact order of tribulation events. Perhaps Christ would tell us too: “Open your eyes! Look at what God is doing among you and among the nations of the world!”
 There is no relevant textual variant in 17:21, although manuscript D adds “don’t believe it” before “for behold the kingdom of God…”
 The title translates as: The problem of the delay of parousia in the Synoptic Gospels, ISBN: 9783112325995; a later edition included a study of the book of Acts. Das Problem has not been translated into English.
 If we look at Luke 11:20, the text of p75 (late 2nd or early 3rd centuries) is identical to that of Nestlé-Aland 28, including the use of the aorist tense of ἐφθάσεν/ephthasen, which has a perfective meaning of “[the kingdom of God] has arrived.”
 See especially how N. T. Wright develops this trend in Paul: A Biography.
“God’s Kingdom – right in front of your nose! Luke 17:21,” by Gary S. Shogren, PhD en New Testament Exegesis, Professor at Seminario ESEPA, San José, Costa Rica