Because the Puritan era was already on the wane in 1630 when he began writing Of Plymouth Plantation, Bradford wanted to make sure that neither the history of the journey on the Mayflower in 1620, nor the early years of the Massachusetts Bay Colony were forgotten by future generations.
Bradford believed that the Separatists, as they called themselves, were carrying out God's will by leaving England (and the Netherlands, where they had lived for around ten years) and establishing a new Zion in America. He saw parallels between biblical stories and the trials they faced in Massachusetts and wanted to leave an account to inspire succeeding generations to maintain their beliefs and practices.
In the journals that informed his narrative, Bradford gave very little credit to the people of the Massachusetts Bay Colony for their courage, perseverance, or intelligence. Any positive outcome (i.e., anyone being spared an illness or untimely death) was credited to God, such as the miraculous survival of John Howland, who fell overboard from the Mayflower, and the serious illness Bradford himself suffered in the winter of 1620.