Paul ended his historical review with an exhortation and appeal to his readers (cf. Acts 13:15). He now addressed his two types of hearers collectively as "men brethren" ( Acts 13:38, Gr. andres adelphoi). When it comes to responding to the gospel, all people, Jews and Gentiles, are on the same level. Through Jesus, Paul asserted, everyone who believes (the only condition) has forgiveness of sins (cf. Acts 2:38; Acts 10:43) and justification (God"s judicial declaration of righteousness, cf. Deuteronomy 25:1). Justification could not come through the Mosaic Law, he reminded his hearers. This is the only reference in Acts to justification by faith in Jesus. [source][source][source]
"What we have in the application of Paul"s message (despite its cumbersome expression in its prcis form) are his distinctive themes of "forgiveness of sins," "justification," and "faith," which resound in this first address ascribed to him in Acts just as they do throughout his extant letters." [1][source]
Paul later developed the truth of justification and forgiveness apart from the Mosaic Law in his epistle to the Galatians. He probably wrote Galatians to the same people he spoke to here shortly after he completed this first missionary journey. Later he set forth these themes more fully in his epistle to the Romans. These verses summarize the arguments of Galatians and Romans in one sentence. [source][source][source]