Somebody asked Imam Syed Muhammad Mahdi al-Mau'ood Ahs to explain the purport and essence of all the divine speech (Kalamullah-Allah's Books) in one sentence.
The Imam Ahs said "I will explain the purport and essence of all the divine Books-The Old Testament, The Psalms of David, The Bible and The Quran- in one sentence, and that sentence is La ilaha illa Llah (There is no god but God-Allah).
(1. Hashia Insaaf Nama - Hashia Sharif)