Complex sentence: He's regarded as one of the most important Shia "Mujtahid" at the time and is known for using his position as a marja for political use in the Persian Constitutional Revolution (1905-1911).
Split into simpler sentences: He's regarded as one of the most important Shia "Mujtahid" at the time. He is known for using his position as a marja for political use in the Persian Constitutional Revolution (1905-1911).