When they had preached the gospel to that city (euaggelisamenoi tēn polin ekeinēn). Having evangelized (first aorist middle participle) that city, a smaller city and apparently with no trouble from the Jews.
Had made many disciples (mathēteusantes hikanous). First aorist active participle of mathēteuō from mathētēs, a learner or disciple. Late verb in Plutarch, to be a disciple (Mat 27:57 like Joh 19:38) and then to disciple (old English, Spenser), to make a disciple as in Mat 28:19 and here. Paul and Barnabas were literally here obeying the command of Jesus in discipling people in this heathen city.
They returned to Lystra and to Iconium, and to Antioch (hupestrepsan eis tēn Lustran kai eis Ikonion kai eis Antiocheian). Derbe was the frontier city of the Roman empire. The quickest way to return to Antioch in Syria would have been by the Cilician Gates or by the pass over Matthew. Taurus by which Paul and Silas will come to Derbe in the second tour (Acts 15:41-16:1), but difficult to travel in winter. But it was necessary to revisit the churches in Lystra, Iconium, Antioch in Pisidia and to see that they were able to withstand persecution. Paul was a Roman citizen though he had not made use of this privilege as yet for his own protection. Against mob violence it would count for little, but he did not hesitate. Paul had been stoned in Lystra, threatened in Iconium, expelled in Antioch. He shows his wisdom in conserving his work.