Acts 11:17

πιστεύσασιν, who believed. The participle refers alike to the preceding αὐτοῖς and ἡμῖν, and thus the two cases are made parallel exactly as in the narrative of Acts 11:15. For just as in the case of Peter and the Apostles, their faith was existing before the gift of the Spirit, so in Cornelius and in his companions there existed a degree of faith, or there could have been no sincere prayer offered by them.

ἐγὼ τίς ἤμην δυνατὸς κωλῦσαι τὸν θεόν; who was I that I could withstand God? There are in reality two questions here merged into one. Who was I? Was I able to withstand …? So also Luke 19:15 τίς τί διεπραγματεύσατο = who had traded, and what he had made thereby.