Why is the Shahadah important?
it's a declaration of faith, belief in Tawhid.
Is expressed in the six beliefs and five roots of Islam (in Tawhid, Nubuwwah and Imamah)
Is repeated multiple times a day during Salah; is believed to be the most important pillar of Islam; first words whispered into a baby's ears, is recited on a Muslim's deathbed
"There is no god but Allah and Muhammed is the messenger of Allah" (for Shi'as: "...and Ali is the representative of Allah"