Jews have simple faith because the heavenly part of the soul sees the divine. The source of faith is the connection of the soul’s root to the divine.

There are two reasons why Jews believe in God with simple faith and do not require proof in this matter. [The first is] because “their mazal sees” (Sanhedrin 94a): The higher soul sees the divine (with perception that is beyond the intellect), and this inspires faith in God within the soul that inhabits the body. A further explanation is that the root of faith lies at the essence of the soul (which is higher than the level of “their mazal sees”). This essence of the soul is inherently tied to the divine [in that it itself is divine], in a manner that is independent of causation and does not depend on perception [of any sort,] even perceptions that are beyond the intellect. (The Lubavitcher Rebbe, Torat Menaĥem, Sefer HaMa’amarim Melukat 3, p. 34)