Holy Books do not teach a person whether God exists. The only way to detetmine that is through yoga. Specifically one adopts the proces of satya-advaita yoga. This is the certain way to erase all doubts that God exists. In this form of yoga one is always studying everything by oneness with truth. One probes everything that moves.
When Realisation is complete only then does probing stop. One then has become a spiritual Master. The person is finally to be known to be 'Mature' knowing exactly what he is doing and in full command of his mental faculties.