Yes, all the Prophets preached monotheism. The message with which all the Prophets were sent was that of pure monotheism, emphasizing the belief in one God. Throughout history, every Prophet has conveyed this fundamental truth, calling their communities toward the worship of the one true God. The mission of all the prophets right from Adam to Christ was one and the same—of establishing the ideology of monotheism in the world, so that man may worship one God alone. As we know, there came a large number of prophets in ancient times, but the message of monotheism remained the main doctrine of each Messenger. Despite the challenges they faced, their mission consistently focused on establishing monotheism as the core of their teachings.

Source: Islam Rediscovered