2. How the apostles abandoned and rejected the unbelieving Jews (v. 51): They shook off the dust of their feet against them. When they went out of the city they used this ceremony in the sight of those that sat in the gate; or, when they went out of the borders of their country, in the sight of those that were sent to see the country rid of them. Hereby, (1.) They declared that they would have no more to do with them, would take nothing that was theirs; for they sought not theirs, but them. Dust they are, and let them keep their dust to themselves, it shall not cleave to them. (2.) They expressed their detestation of their infidelity, and that, though they were Jews by birth, yet, having rejected the gospel of Christ, they were in their eyes no better than heathen and profane. As Jews and Gentiles, if they believe, are equally acceptable to God and good men; so, if they do not, they are equally abominable. (3.) Thus they set them at defiance, and expressed their contempt of them and their malice, which they looked upon as impotent. It was as much as to say, "Do your worst, we do not fear you; we know whom we serve and whom we have trusted." (4.) Thus they left a testimony behind them that they had had a fair offer made them of the grace of the gospel, which shall be proved against them in the day of judgment. This dust will prove that the preachers of the gospel had been among them, but were expelled by them. Thus Christ had ordered them to do, and for this reason, Matt 10 14; Luke 9 5. When they left them, they came to Iconium, not so much for safety, as for work.

Matthew Henry Commentary on the Whole Bible (Concise) (Matthew Henry)

Verses 42-52 The Jews opposed the doctrine the apostles preached; and when they could find no objection, they blasphemed Christ and his gospel. Commonly those who begin with contradicting, end with blaspheming. But when adversaries of Christ's cause are daring, its advocates should be the bolder. And while many judge themselves unworthy of eternal life, others, who appear less likely, desire to hear more of the glad tidings of salvation. This is according to what was foretold in the Old Testament. What light, what power, what a treasure does this gospel bring with it! How excellent are its truths, its precepts, its promises! Those came to Christ whom the Father drew, and to whom the Spirit made the gospel call effectual, ( Romans 8:30 ) . As many as were disposed to eternal life, as many as had concern about their eternal state, and aimed to make sure of eternal life, believed in Christ, in whom God has treasured up that life, and who is the only Way to it; and it was the grace of God that wrought it in them. It is good to see honourable women devout; the less they have to do in the world, the more they should do for their own souls, and the souls of others: but it is sad, when, under colour of devotion to God, they try to show hatred to Christ. And the more we relish the comforts and encouragements we meet with in the power of godliness, and the fuller our hearts are of them, the better prepared we are to face difficulties in the profession of godliness.