We only know that to be the case if we have faith. And yet, even with faith, we as humans do have weaknesses and can 'cave in' to temptations. Istighfar.
It is hard to convince people who do not want to be convinced. The main problem I have noticed in my discussions with people is the fact that those with issues tend to 'humanise' God and are unable to grasp the concept of 'Hayyul Qayyum', the concept of Qada and Qadar and ar-Rahman ar-Rahiim (citing the sufferings to show that He is not).
It can get frustrating when the issues jump from point to point, often in isolation, on their doubts. Islam is absolute. You either believe or you don't. There is no half way to Jannah.
"Surely! Allah wrongs not even of the weight of an atom (or a small ant), but if there is any good (done), He doubles it, and gives from Him a great reward." 4:40
"Whosoever intercedes for a good cause will have the rewardthereof, and whosoever intercedes for an evil cause will have a share in its burden. And Allah is Ever All-Able to do (and also an All-Witness to) everything." 4:85
Peace