The ruling to believe is a legal ruling (hukm shar’i) not a rational ruling.
The first obligation and the first necessary act of the believer is to actively know Allah and have gnosis (ma’rifa) of the existence of Allah. This includes to know what is necessary, possible, and impossible for Allah. All the scholars agree that the worship of one is invalid if one does not know who He is worshipping. This is the majority opinion, but other scholars have differed about this, som esay the first obligation is to actually contemplate the knowledge of Allah and His attributes first for example.
Gnosis (ma’rifa) in this context means: absolute certain belief, which agrees with reality, and is supported by evidence.
Absolute certain belief: to have total conviction about something, without any presence of any little doubt.
In agreement with reality: to have total conviction in something that is not inaccurate or erroneous, like believing in the pre-eternality of the world.
Supported by evidence: to acquire evidence either rationally (aqli), through the physical senses (hawas), true reports (al-khabar as-sadiq), or the illumination of the soul (ishraq al-ruh). The common believer can rely on general evidences, and the theologian develops proofs as specific evidences. See: Types of Evidence (Daleel).
An important issue in terms of knowing Allah is that one must generally do so without the blind following of others. This applies to knowing Allah, and in other matters of faith too. This concept is called blind following (taqlid) which is one who: acquires certain belief in the existence of Allah and His attributes on the statement of another person without any evidence or proof. This person is called a conformist (muqallid). See: The Conformist (Muqallid).