Kasravi called for enlightenment in all aspects of life and for a national struggle against "intellectual and scientific backwardness." He criticized what he called the "living conditions, superstitions and social customs" of the Iranian people. Kasravi's stances against religious institutions, traditional values, and his attack on religious and cultural beliefs, including the Shia Muslim and Baha'i faiths, were among the reasons for the opposition against him.

Kasravi became a cleric at the age of 20. He had memorized the Quran and was a leader of Friday prayers. In 1912, Kasravi supported Iran's constitutionalist movement, which advocated for the country's absolute monarchy to be replaced with a constitutional arrangement, resulting in other clerics insulting and excommunicating him. He resigned. In 1915, Kasravi learned English and Esperanto as he taught Arabic to students at the American School of Tabriz, also known as the Memorial School.

Two years later, he became an employee of the judiciary at the suggestion of the head of the Azerbaijani Court of Appeals. He was later appointed head of the judiciary in the provinces of Mazandaran, Damavand, Zanjan and Khuzestan. In 1926, Kasravi became an inspector and head of one of Tehran's courts. In the same year, he learned the ancient Pahlavi (or "Middle Persian") language from Professor Herzfeld who came to Tehran to translate "Kar-Namag i Ardashir i Pabagan" (the Book of the Deeds of Ardeshir, Son of Papak) from Pahlavi into Persian.

In the winter of 1932, he wrote a bold letter to Reza Shah, calling the judiciary a hotbed of bribery and corruption, and withdrew from it. He then published the first part of the book “Aeen” (Religion), which was a pivotal change in Kasravi's life, from researcher and linguist to theorist and social reformer. As he continued his  research, for nine years he published the monthly "Peyman" newsletter from December 22, 1933. From February 1941, he founded “Parcham” newspaper and published it until 8 December 1942, when all Tehran newspapers were shut down.

In February 1943, a case was filed in court against Kasravi by a group of clerics and businessmen because of Kasravi’s research against religious ignorance and superstition. On April 28, 1945, Mojtaba Mir-Lohi, also known as Navvab Safavi, a member of the Fada'iyan-e Islam group, which included extremist Muslims and terrorists, attacked Kasravi near his home and shot him twice in the back.

Kasravi survived the attack. Navvab Safavi was later released through the mediation of some businessmen and clerics. On March 11, Navvab Safavi, along with other members of the Fada'iyan-e Islam group, attacked Kasravi once more at the Palace of Justice, this time killing him along with two of his colleagues. Kasravi was 53 years old.

Kasravi's work included over 70 books that have been translated into different languages. Among his important works are books "History of the Iranian Constitution" and "Eighteen-Year History of Azerbaijan", which are among the most important works in the history of the Iranian constitutional movement. In his book "Azeri, or the Ancient Language of Azerbaijan", he first proposed the theory that the historical language of the region of Azerbaijan, before the spread of the Azerbaijani Turkish language, was of an Iranian language family. This theory is fully accepted among linguists today.