The theory that Christianity was a major factor in the collapse of the Roman Empire was first published in 1776 by

Marquis de Condorcet was a French philosopher and mathematician known for his work in social and political theory, but he did not specifically address the decline of the Roman Empire or link it to Christianity in his writings. His focus was on progress and the potential for human improvement rather than historical decline.

Gotthold Ephraim Lessing was a German playwright and philosopher who contributed to the Enlightenment but did not focus on the Roman Empire's collapse in relation to Christianity. His works emphasized rationalism and religious tolerance, rather than the historical analyses that Gibbon provided.

James Watt was an inventor and mechanical engineer, best known for his improvements to the steam engine. His contributions were primarily in the field of industrial technology and did not involve historical analysis of the Roman Empire or Christianity's role in its decline.

James Boswell was a biographer and diarist, most famous for his biography of Samuel Johnson. While he made significant contributions to literature, he did not write about the historical factors behind the fall of the Roman Empire or the influence of Christianity.