People who reject religion and God often do so because they cannot understand the existence of pain and suffering in this world. They expect this world to be a place of no evil, and when they find any evil, they use it to reject the place of God and religion. “Why would God not create a perfect, blissful world, without pain, without suffering, without death?” they ask. “How could a merciful God create an imperfect world?”
The sad irony is that we Muslims as well essentially believe in the exact same thing. Indeed, a true God would create a perfect world, and it’s called Jannah. These people want to be given Jannah without any effort. They believe they have essentially ‘earned’ Jannah merely by their existence (and of course even their existence was not even of their own doing).
We, on the other hand, believe the pain and suffering and evil that exists in this world is all meant to extract from us the potential to counter than pain with mercy, to counter that suffering with compassion and patience, and to counter that evil with good. Each opportunity, if approached with faith, becomes an opportunity of good, and any pain or suffering diminishes when compared to the good that comes out of it. And in the end, the greatest of all good, which is Paradise, is contingent on our responses to these trials of this world.
Rather than question the existence of God because of calamity (which is similar to Satan rejecting the worship of Allah because of his own arrogance), a humble believer submits to the Will of Allah and recognizes that every calamity has a greater good hidden in it, and every believer has the potential and capacity to unlock that greater good and maximize the potential to earn Paradise, the place of no suffering or evil.