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

Condorcet, a French philosopher and mathematician, focused on ideas of progress and democracy, rather than on the historical analysis of the Roman Empire's collapse. His works did not specifically address the role of Christianity in this context, making him an incorrect option.

Gotthold Ephraim Lessing was a playwright and philosopher known for his contributions to literature and religious tolerance. While he engaged with themes of religion, his works did not delve into the historical analysis of the Roman Empire's decline in relation to Christianity.

James Watt was an inventor and mechanical engineer, best known for his improvements to the steam engine. His contributions were in the realm of technology and industry, with no significant writings concerning the fall of the Roman Empire or the impact of Christianity on it.

James Boswell was a biographer and diarist, most famous for his biography of Samuel Johnson. His works primarily focused on literature and personal narratives, rather than historical theories about the Roman Empire or Christianity's role in its decline.