38 Therefore let it be known to you, brethren, that through this Man is preached to you the forgiveness of sins;
39 and by Him everyone who believes is justified from all things from which you could not be justified by the law of Moses.
40 Beware therefore, lest what has been spoken in the prophets come upon you:
41 ‘Behold, you despisers, marvel and perish! For I work a work in your days, a work which you will by no means believe, though one were to declare it to you.'”
through this Man (v.38) — through Him and no other
[Paul’s message] corresponds in different ways to Peter’s preaching to the Jews, yet it differs from Peter’s message in regard to the Gospel. As we have seen Peter’s preaching was addressed to the Jews and he offered forgiveness of sins to them who repent and are baptized. But Paul utters a truth for the first time, which Peter did not declare. He said: “By Him all that believe are justified from all things, from which ye could not be justified by the law of Moses.” It is justification by faith he now preaches. We have in his first recorded utterance the Keynote of the two great Epistles, dictated by the Holy Spirit to Paul; the great doctrinal Epistle to the Romans and the controversial Epistle to the Galatians, the defense of his Gospel.
He did not once mention the word repentance. He has nothing to say of baptism. He did not exhort them to do the best they could or to live under that law, which they had. The Law of Moses could not justify them. The Epistle to the Galatians, most likely read by those in Antioch who heard Paul speak, enlarges upon this fully. Justification from all things is offered now by faith in Jesus Christ. All that believe are justified of all things is the blessed message of the Gospel of Grace which Paul preached and which is not after man, but by the revelation of Jesus Christ (Galatians 1:11-12) — Gaebelein, pages 241-242, 245.
The quote in verse 41 is from Habakkuk 1:5.
despisers (v.41) = look down upon, think lightly of
We find the law and the prophets referred to at both the beginning and the end of this synagogue service (vs. 15, 39-40). At the beginning the law and the prophets are read and Paul and Barnabas are asked if they have some word of exhortation for the people. Paul has a word of exhortation from both the law and the prophets. After showing how the Savior, though crucified and rejected by His people, is alive again, he exhorts his hearers: Through Christ they may enjoy the forgiveness of sins, for by Him all that believe are justified from all things from which they could not be justified by the law of Moses.
And to this the apostle adds a final exhortation not to despise the gracious offer, lest they find themselves in the position of Christ-rejecting Israel. “Beware lest that come upon you” or, “lest you find yourselves in the situation,” of those to whom God must now speak in the same way as He had spoken in Habakkuk 1:5 (though with regard to a different matter).
How applicable were the words of Habakkuk 1:5 to Israel’s situation at this time! They had despised God’s goodness, now God was doing a work which they could not believe. They said in effect: “But He cannot save the Gentiles apart from us! What about all the Old Testament promises?” Yet He was blessing the Gentiles apart from them — through the Christ whom they had rejected, and they could only “Behold … and wonder, and perish.” Now Paul exhorts, especially his Jewish hearers, to beware lest that come upon them.
And thus the Spirit-led apostle used the opportunity to exhort his hearers from both the law and the prophets. From the law: “Do not trust in the law; trust in Christ.” From the prophets: “Do not despise the gracious invitation.” — Stam, pages 202-203