"They were very angry." The Sadducees, in particular, would have been angry about what Peter and John were saying because they did not believe in resurrection.
Peter and John were saying that God would raise people from the dead in the same way as he had raised Jesus from among the dead. Translate this in a way that allows "the resurrection" to refer to both Jesus's resurrection and the general resurrection of other people.
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.
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