The Communist Party of India (Marxist) is atheistic by ideology. There is no credible evidence to verify whether its leaders are atheists or agnostic.
CPI(M) is guided by the principles and philosophy of Marxism-Leninism, which stands to end the exploitation of man by man.
In 2010, The Hindu reported Prakash Karat, former General Secretary of CPI(M), saying, "Marxists are atheists, i.e., they believe in no religion. But Marxists understand the origin of religion and the role it plays in society. While the CPI(M) upholds the materialist outlook, it does not bar people having religious faith from joining the party." Karat himself is alleged to be an atheist. Business Standard reported a census in 2014 stating that only a few parties like the CPI(M) and CPI claim to be irreligious. It was stated in comparison to other avowedly atheist parties like the RSP, the DMK, and the Maoists. Current General Secretary of CPI(M), Sitaram Yechury, told Hindustan Times in 2018 that, "I was named Sitaramarao, after my grandfather, but I dropped the caste appendage and carried on with my name."
There have been many CPI(M) leaders over the decades, and there is no definitive evidence to verify all of their religious beliefs or lack thereof.