At the Jerusalem Council (Acts 15), Peter cited what God did through him in Caesarea in bringing Gentiles to salvation (15:7-11) as definitive proof that Gentiles did not need to be circumcised and keep the Law of Moses to be saved (cf. 15:1, 5). In his citation, Peter made a remarkable statement:
“And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as He did unto us” (Acts 15:8).
Here, Peter declared that God, who infallibly knows the hearts of all people, supernaturally testified to the saving faith of the people in Caesarea to whom Peter preached the gospel (10:31-48). God did so by giving them the Holy Spirit.
Based on this statement, Peter and the other six witnesses who were with him (11:12) had absolute certainty that Cornelius and the rest of Peter’s hearers on that occasion had been saved. Moreover, Cornelius and all who were with him themselves thus had absolute certainty about their own salvation.
God supernaturally gave these people such certainty, and they would have never needed to have any doubt about their own salvation. Although God does not do so for us today with supernatural testimony that is outwardly visible, we, too, individually can and should have the same certainty about our own salvation through the ministry of the Spirit of God in our own hearts (Rom. 8:16; Gal. 4:6).