Ibn Taymiyyah (رحمه الله) said: “The Salaf and the leading scholars are agreed that the first of which the servants are commanded with are the Shahādatān, and that whoever does that before maturity is not commanded to renew them following maturity.” Darʾ al-Taʿāruḍ (8/11)

Ibn al-Qayyim (رحمه الله) said: “Tawḥīd is the first of that by which Islām is entered into and the last of that by which the world is departed from.” Zād al-Maʿād (2/22).

Ibn Kathīr (رحمه الله) said: “The first obligation upon the servants is to know that there is none worthy of worship but Allāh alone, without any partners.” Tafsīr al-Qurʾān al-Aẓīm (5/277).

Shaykh al-Fawzān (حفظه الله) said: “The first obligation upon the servants is the worship of Allāh. As for knowing him, then they know him through the fiṭrah, the intellects, creational evidences and Qurʾānic evidences.” Sharḥ al-Durrah al-Muḍiyyah (p. 73).

This is in opposition to the people of kalām who claim that the first obligation is “investigation” (into Allāh’s existence by proving the origin of the universe), and some of them even say it is “doubt” which is then to be removed through rational evidences and through which faith is attained. This is false because Allāh’s existence is fiṭriyy (innate, intuitive) and ḍarūriyy (necessary). Further, proving the origin of the universe is not a condition for proving the existence of a Creator, since His existence can be established through other ways.