Why would a Muslim consider the Shahadah to be the most important pillar of Islam?

A Muslim may consider the Shahadah to be the most important pillar of Islam because it established believers. The Qur’an quote, “How shall God guide a people who disbelieved after their belief and (after) they had borne witness”, suggests if a Muslim does practice the Shahadah after they are whispered it at birth, their faith is unconsolidated. Ergo if someone were to undervalue, neglect, or say the Shahadah with insincerity, Allah would come to believe they their faith was insincere. Without this belief, even if someone practice Salat and Sawm, they would still, in the eyes of Allah, not be Muslim. Therefore Muslims consider the Shahadah to be the most important pillar of Islam as it ensures they are legitimate Muslims.