As a result of Stephen’s death, there breaks out a great persecution against the Church; which results in a great dispersion of the Church; which results in widespread evangelism by the Church. Despite the attempts to smother the gospel, the gospel spreads further still. Like trying to blow out a fire but instead only spreading sparks further outwards, persecution of the Church only spreads to further its witness in the world.
Stephen’s death was not in vain because a life lived for Christ, and a life given for Christ, is never a life lived or given in vain. As the Apostle Paul, who was present at Stephen’s death, would write years later: “To live is Christ; to die is gain.”