What is the way to get rid of the whispers of atheism that raise doubts about the existence of God and the Afterlife, and weaken faith, coupled with the fear of dying outside of Islam?

These thoughts are satanic whispers (waswasa), and they do not harm you as long as you detest them and are averse to them. You must turn away from them and strive against your soul to repel them until they completely disappear. Striving against these whispers is a sign that you are upon goodness, and they do not affect the soundness of your faith.