How does faith in God lead to a sense of tranquility and serenity, and how does prayer contribute to combating psychological illnesses?

Allah has given a parable illustrating the relationship between faith and the tranquility of the heart and the serenity of the soul. He likened the polytheist to a man who has conflicting partners, and the believer to a man who belongs to one master. Thus, the believer has a stable heart, while the polytheist is agitated. Faith, through the knowledge of Allah and His names and attributes, establishes tranquility in the heart.

The pillars of faith contribute to tranquility. Belief in the Last Day fosters detachment from this world, and belief in divine decree brings serenity. Allah Almighty rewards the believer with security and tranquility in this world and the Hereafter. Prayer, as the greatest form of practical faith, provides psychological comfort and a cure for psychological ailments, and it is the refuge of the believer during hardships.