The Apostle Paul gives us the most detail of his encounter with the Lord Jesus Christ (Acts 9) as he stands before king Agrippa (Acts 26). It is in Paul's defense that the Apostle reveals the purpose of his ministry and his witness by the Lord's own words: "...to open their eyes, and to turn them from darkness to light, and from the power of Satan unto God, that they may receive forgiveness of sins, and inheritance among them which are sanctified by faith that is in me" (Acts 26:18).
"Faith that is in me" is faith that is in the Lord Jesus Christ! It is faith in his name (Acts 3:16). To Jesus give all the Old Testament prophets witness, that "through his name whosoever believeth in him shall receive remission of sins" (Acts 10:43). It is faith in his person, in his actions: his death, his burial and his resurrection!
Such faith has power to give us (1.) forgiveness of sins and (2.) an inheritance among "them that are sanctified by faith that is in me." Who are those saints? Hebrews 11 gives us a catalogue of many of them. It is God's will that we are sanctified through the offering of the body of Jesus Christ once for all (Hebrews 10:10).
Christ died once: "for by one offering he hath perfected forever them that are sanctified (Hebrews 10:14). He died for all (the whole world - 1 John 2:2). He died for Adam, he died for me, and he died for you. Through faith in him, you can have forgiveness of your sins and an eternal inheritance with all who are trusting the Lord Jesus Christ! That is the POWER of FAITH in JESUS CHRIST!