Hojjat al-Islam literally means "Proof of Islam." Hojjat al-Islam began as an honorific title given to high-ranking scholars (ulema) in both Sunni and Shi˓ite Islam. Hence al-Ghazali (d. 1111) was given the title Hojjat al-Islam, to signify his skill in arguing for the truths of Islam. It appears to have remained a general term of respect for a scholar. In the nineteenth century, the title began to reflect the more hierarchical structure of the Shi˓ite seminary system. At first, scholars like Muhammad Baqir al-Shafti (d.1844) were given the titles mujtahid, Ayatollah, and Hojjat al-Islam. Later usage of the term Hojjat al-Islam was restricted to scholars of a rank lower than Ayatollah. A Hojjat al-Islam, since the Islamic revolution in Iran, is an "aspiring Ayatollah" who has completed his bahth-e kharij (the highest level of formal instruction) and is teaching, but has not yet gained sufficient prestige to be regarded as Ayatollah. While both Ayatollah and Hojjat al-Islam were titles of distinction in the late nineteenth and early twentieth centuries, the titles have become relatively common in recent years, and this may reflect either a lowering of the qualification threshold, or an improvement in educational techniques in the Shi˓ite seminaries of Mashhad, Qum, and the Atabat.
Mottahedeh, Roy P. The Mantle of the Prophet: Religion and Politics in Iran New York: Simon and Schuster, 1985.