How can one get rid of thoughts that insult the Divine Self and those mocking inner voices, and deal with questions such as the fate of the world that did not know Islam?

What goes on in the soul without being spoken is forgiven. The treatment for obsessive thoughts is to ignore them and distract oneself with beneficial activities, to persevere in prayer and ذكر (remembrance of Allah), and to associate with righteous people. Psychological anxiety is treated with ذكر (remembrance), supplication, and good deeds. Magnifying and rejecting obsessive thoughts is a sign of faith. One must believe in Allah Almighty's justice and submit to Him. The disbeliever was created to worship Allah and affirm His Oneness, and blessings were showered upon him so that he might submit. Whoever benefits creation but neglects the worship of the Creator goes astray; he will be rewarded for his good deeds in this world and punished for shirk (associating partners with Allah) in the Hereafter. Whoever hears about Islam and does not submit is among the people of Hellfire.