Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
The thoughts and intrusive whispers that come to a person concerning belief in the unseen do not necessarily mean that he has denied the creed or left the faith. Such whispers may be caused by Satan’s manipulation or by inner psychological disturbance, and a Muslim is not held accountable merely for thoughts that he dislikes and does not believe.
Therefore, you should not dwell on these thoughts or try to resolve every intrusive thought through repeated thinking, because dwelling on them may make them stronger and increase confusion. When doubts about the reckoning, Paradise, or Hell come to you, seek refuge in Allah, turn away from them, and occupy yourself with what benefits you in worship, knowledge, and righteous deeds.
Reflect on the wisdom behind the existence of the Hereafter. Complete justice is not fully manifested in this worldly life. A wrongdoer may die without receiving his punishment, while a person who has been wronged may die without obtaining his right. Among the manifestations of Allah’s perfect justice is that there be another life in which the righteous are rewarded for their righteousness and the wrongdoer is punished for his wrongdoing.
Likewise, the pleasures of this world are limited, come to an end, and eventually become familiar to a person, whereas the bliss of the Hereafter never ends and is never accompanied by boredom. Allah تعالى says:
﴿As for those who believe and do righteous deeds, We will admit them into Gardens beneath which rivers flow, where they will remain forever.﴾
[An-Nisa: 57].
The basic principle is that belief in the Hereafter is part of belief in the unseen, which Allah and His Messenger ﷺ have informed us about. A person is not required to comprehend with his intellect all the details of the world of the Barzakh, Paradise, and Hell, nor to know how the passage of time occurs therein. These are matters of the unseen. We believe in what has been conveyed by revelation and do not burden ourselves with matters for which there is no evidence.
As for experiencing these intrusive thoughts while maintaining your prayers and supplicating to Allah تعالى, this does not mean that your supplication has not been answered. The answer may be that Allah helps you resist them, turns their harm away from you, or causes them to increase your dependence upon Him. You may also need patience and persistence in seeking knowledge and engaging in worship.
Therefore, do not allow these thoughts to become a reason for abandoning worship or falling into despair. Rather, remain steadfast in prayer, read the Qur’an frequently, contemplate the verses concerning promise and warning and the Hereafter, keep the company of people of knowledge and righteousness, and read books on strengthening faith and the causes of its increase, while turning away from doubts and material that intensifies intrusive thoughts.
Most importantly: do not treat a passing thought as though it were a firmly held belief. A person may experience thoughts that he dislikes and does not accept, without actually believing in them.
And Allah تعالى knows best.