Thank you my dear friend for this very good question!  The short answer to your question is that this shaking off of the dust of the disciples’ feet was a sign of God’s dissatisfaction towards those of Israel who, by rejecting the message of the disciples were actually rejecting their Messiah.  In one of the Scriptures which you have cited (Luke 9:2 and 5 we read: “And He sent them to preach the kingdom of God, and to heal the sick…; And whosoever will not receive you, when ye go out of that city, shake off the very dust from your feet for a testimony against them….”)

Perhaps it might help to define what is meant by the term “the Kingdom of God”. The Lord Jesus, when He began His earthly ministry, was preaching that the kingdom of God was at hand. The King, the Messiah of God, had entered the world.  The Lord Jesus, in the verses you have cited, sent forth His disciples to preach the gospel of the Kingdom given the coming of the Messiah to the children of the house of Israel. Now, however, while some received this message with gladness and believed, others did not; as we read in John 1:11, “He came unto His own, and His own received Him not.” When the Christ of God was ultimately rejected by His people, He went forward to accomplish the mission for which the Father had purposed for Him from before the dawn of time, and that was that He must die on the cross in payment for our sins, so that all who received Him might be saved (John 1:12; John 3:16).

So, given the rejection of the Messiah by His own people in Israel, what became of the Kingdom of God? Well, the kingdom will be established in its fullness when the Lord Jesus returns to the earth in power and in glory. But, until He does return, today is the age of the church, and the church in its universal sense is the body of true believers in Christ Jesus. The Morrish Bible Dictionary explains the relationship of the church and the Kingdom of God as follows: “The kingdom is the sphere of Christ’s rule; whereas the church is the dwelling place of God by the Spirit. Neither will the duration on earth of the church and the kingdom be the same; the kingdom will be set up in power after the rapture of the church and will continue during the millennium. The Christian, besides sharing in the privileges of the church, has also the privileges and responsibilities attaching to the kingdom…. His place in heaven is by grace apart from his works, but his reward in the kingdom will be according to his faithfulness to his Lord.”

Now, just before this all happens, and I believe it will not be a long time from now, the Lord Jesus will return to the clouds to take His children, all born again Christians, home to be with Himself (1 Thessalonians 4:16-18). Then there will be 7 years of tribulation in Israel (Daniel 9:27), and after that, Christ Jesus will come back to the earth in power and in glory to overthrow all His enemies, and in that day He will visibly establish the Kingdom of God, ruling the whole earth in righteousness. Now in that latter day, there will be very serious consequences indeed for those who reject the coming King of Glory; there will be severe judgment in that latter day for those who scorn their King, and thus we see the sign of shaking off the dust of the feet of those bearing the good news of the coming King as a sign of the indignation of God. On the other hand, there will be blessing and restoration for Israel for those who faithfully receive their King, and the faithful of Israel will receive their King in that day for sure (Isaiah 53).

Now, what does this all mean for born again Christians today? We preach the Gospel of Jesus Christ, salvation by Grace through faith alone. For those who reject this wonderful and gracious Gospel message and do not repent of this rejection during their lives, there is the wrath and indignation of God facing them (Romans 2:8; Revelation 20:12-15). Now indeed you might ask, “if we present the Gospel of salvation to someone and they will not hear, repent and believe, are we to shake off the dust from our feet as a testimony against them”? Well, in Acts 13, when Paul and Barnabas preached the Gospel of salvation to the Jews in the synagogue at Antioch in Pisidia and were not received, this is precisely what they did (Acts 13:51 where we read: “But they shook off the dust of their feet against them, and came unto Iconium.”). But this was a sign against the unbelieving Jews in that city who stirred up trouble for these Gospel preachers. The Gospel was to be preached to the house of Israel first (Acts 1:8). But these in Antioch of Pisidia judged themselves “unworthy of everlasting life;” so Paul and Barnabas proclaimed that they would then take the message to the Gentiles, many of whom did receive it with gladness. Again, this shaking off the dust was a sign of God’s indignation against the unbelievers of the house of Israel. I don’t know that born again Christians today would visibly shake off the dust of our feet in this way, but clearly the instruction of our Lord Jesus in the verses you have cited does underscore the fact that it is a very serious matter indeed for anyone to reject the Gospel of Jesus Christ. We, as true Christians, are to sow the precious seed of the Gospel in love. Today is the day of Grace, and God desires that all who will believe have the opportunity to come to salvation, and it is for this very reason that the day of judgment has been delayed for a time (2 Peter 3:9; John 3:16). We might find that our message is rejected, but we should, I believe, always pray for the lost and continue to spread that precious seed in hope because we do not know the work of the Holy Spirit which might be going on in the hearts we are preaching to.  It is not certain that those today who once reject the Gospel will have the opportunity to repent in this life (Genesis 6:3), but, those who are ordained to eternal life will certainly come at some point in their lives (Acts 13:48; John 6:37), and what a joy it will be in the glory for those who have sown that precious seed with many tears, as they come rejoicing, bringing their sheaves with them! (Psalms 126:5,6).  (SF)  (601.1)