The Four Imams of Aqida - Tahawi, Ashari, Maturidi & Ibn Hanbal by Sheikh Hamza Yusuf

Doubt of God is a modern concept.  The proof of the existence of God is to build the foundation of belief.

Facing the mass confusion in Muslim lands, great Muslim scholars (Tahawi, Ashari, Maturidi) emerged to formulate the creed (Aqida) to save Islam in 9^th century.

Aqida: what is the conviction about.

The balance of Aql  (Reasoning/Understanding) and Naql (Text/literalist reading). The man who combine Aql and Naql together is Iman Ghazali.  

Three levels of Faith: Iman, Islam, and Ihsan