"But" indicates a strong contrast between what the people did and what God did.
"raised him from among those who were dead." To be with "the dead" means that Jesus was dead.
From among all those who have died. This expression describes all dead people together in the underworld. To come back from among them speaks of becoming alive again.
"Raised" here is an idiom for "caused to live again" AT: "caused to live again" (See: rc://en/ta/man/translate/figs-idiom)
This can be stated in active form. AT: "The disciples who traveled with Jesus from Galilee to Jerusalem saw him for many days" (See: rc://en/ta/man/translate/figs-activepassive)
We know from other writings that this period was 40 days. Translate "many days" with a term that would be appropriate for that length of time.
"are now testifying to the people about Jesus" or "are now telling the people about Jesus"