His lived without sin. His death atoned for the sins of all believers. Satan is not a quasi-deity with magical powers. Satan is the principle of evil which resides in people and promotes them to sin and rebel against God. People lose consciousness at death and do not regain it unless they are resurrected at some date in the future.
Salvation requires an adult to both accept the gospel message and be baptized.
Christadelphians do not believe in original sin. This is the traditional Christian concept that every child, at birth, has inherited part of the sin committed by Adam and Eve in the Garden of Eden. After Jesus returns to earth in the near future, the wicked will be destroyed and believers will reign with Christ for a millennium. After that period, those selected for eternal life will live on earth forever. Christadelphians do not believe that the saved will spend eternity in heaven.