What is the difference between the testimony "لا إله إلا الله" and belief in Allah?

Belief in Allah Almighty is the most important pillar of faith. If faith (Iman) is coupled with Islam, then faith is اختصاص to the inner (heartfelt belief), and Islam is اختصاص to the outer (uttering the Shahada and performing the pillars). Both are interdependent; one is not valid without the other. If "belief in Allah" is mentioned alone, it encompasses both the outer and the inner, as indicated by numerous religious texts. Faith is a saying, an action, and a belief of the heart, along with utterance by the tongue and action by the limbs. Whoever does so is a believer (Mu'min) and a Muslim.