Final answer:
The success of the modern evangelical movement in the United States is primarily due to its emphasis on personal conversion and evangelism, promoting individual spiritual rebirth and salvation for all.
Step-by-step explanation:
The modern evangelical movement in the United States has been successful primarily because it focuses on personal conversion and evangelism. This movement, particularly during the Second Great Awakening, emphasized the individual's capacity for spiritual rebirth and the belief that salvation is accessible to everyone through personal experience and righteous living. Contrary to rigid Calvinist predestination, evangelicalism offered an egalitarian and optimistic outlook by promoting that any person could achieve salvation. This resonated with a broad swath of society, making evangelical denominations like the Methodist and Baptist churches the largest of their time. Importantly, the evangelical movement also played a role in shaping values and morality within society through moral reform societies and outreach programs, aiming to put individuals on the path to righteousness and improve the human condition via religion and reform.