How can a believer return to faith and rid himself of doubt and ill thoughts of Allah after Allah has tested him and he felt that his prayers were not answered, which caused him to lose the sense of His mercy and led him to question the wisdom of creating humankind for worship, and is there hope of returning to the initial state of faith?

It is part of a Muslim's belief to have faith that creation is Allah's dominion, and He disposes of them as He wills. All of His actions are wisdom, justice, and grace. Allah created creation for the effects of His names and attributes to appear, and for them to worship Him, remember Him, and thank Him. And He, Glory be to Him, loves praise. A believer must strongly repel whispers (waswas), strive in the way of Allah, and know that Allah does not turn away from His servant who turns towards Him. Also, Allah tests the servant to raise his rank and expiate his sins. He may delay answering a supplication for the servant's benefit, or save it for him in the Hereafter. A believer must recite the Quran frequently, and engage in supplication (dua), remembrance (dhikr), prayer (salat), and charity (sadaqah), and strive to seek religious knowledge. Allah's attributes are not limited to mercy; He also has other attributes such as justice and vengeance. And predestination (qadar) is Allah's secret that must be believed in.