Grand Ayatollah Ali al-Sistani, who called on Friday for Iraqis to take up arms against "terrorists", is the country's top Shiite cleric and is revered by millions.
The reclusive Sistani enjoys the kind of following Iraq's Shiite politicians can only dream of, and his call to fight militants who seized swathes of the country this week could give a major boost to recruitment.
"Citizens who are able to bear arms and fight terrorists... should volunteer and join the security forces to achieve this holy purpose," his representative announced on his behalf during Friday prayers in the shrine city of Karbala.
"He who sacrifices for the cause of defending his country and his family and his honour will be a martyr," he added.
The Sunni Islamic State of Iraq and the Levant (ISIL) and allied groups launched an offensive on Monday, taking all of one province and chunks of three more.
Security forces have failed to halt the drive, with some fleeing after throwing away their uniforms and abandoning their positions.
Prime Minister Nuri al-Maliki responded by announcing the government will arm and equip citizens who volunteer to fight, and thousands have turned out at recruitment centres to answer the call.
Sistani, born in the Iranian town of Arshad in 1930, started his religious studies at the age of five, and became a grand ayatollah in 1992.
- Reclusive but influential -
Signup to our newsletter and follow us on Facebook and Twitter!
Despite his huge following, he has generally stayed aloof of Iraqi politics, but has made rare but important interventions since the US-led invasion in 2003.
The elderly cleric repeatedly called for calm during Iraq's brutal sectarian conflict from 2006 to 2008 and threw his weight behind democratic elections.
In late 2003, he demanded that a convention of Iraqis draw up a new constitution and that a transitional government be directly elected by the people, a request the US-led occupation authority was only able to resist through UN mediation.
The following year, Sistani intervened again when an uprising against the US-led occupation by the Mahdi Army militia of anti-US Shiite cleric Moqtada al-Sadr triggered fighting with US troops in the heart of the shrine cities of Najaf and Karbala.
He returned from medical treatment in London to a hero's welcome in Iraq, in time to stop a joint US-Iraqi force from launching a final assault on Sadr's forces who were cornered in the Imam Ali shrine in Najaf.
A tireless proponent of elections since Saddam Hussein's regime collapsed in 2003, the ageing Sistani used his humble quarters in Najaf as a base to guide the country's Shiite majority to power through the ballot box.
After pressuring the US to expedite the path to democratic elections, the cleric was the guiding force behind the creation of a pan-Shiite coalition in Iraq's parliament.
And he has also used his standing among the country's Shiite Arab majority to urge voters to turn out in strength for parliamentary and provincial elections.
But Sistani's decision-making process remains a mystery and little is known about what really goes on in his spartan home in a heavily-guarded alley in the Shiite holy city of Najaf, where he has remained a virtual recluse after years of house arrest during dictator Saddam Hussein's rule.