In the context of OSAS theory I would like to ask you why Ananias and Sapphira, who were husband and wife, instantly died while speaking lies before the disciples. Were not they within the fold of first century believers? Then, why they were not saved even after speaking lies?
There’s no indication in Acts 5:1-11 as to whether Ananias and Sapphira were really saved, nor does it say where they went after dying. Therefore we don’t have enough information to draw a conclusion about their spiritual condition.
But an early or abrupt death is not necessarily an indication of whether a person is saved or not. In other words, a sin in this world can result in the physical death of a believer without affecting his or her salvation. Our behavior can determine what rewards we may or may not receive, but our salvation is based on what we believe, not on how we behave.