I think your premises is wrong here; Islam does not claim world is just ; it ask is followers to pursue Justice and set it to Justice ( which obviously they are not doing) ; if you search Quran you will find numerous verses which talk about setting or trying to setup society toward Justice

Human has been given free will; intellect , revelations , guided through Prophets, innate Fitrah ( to judge goodness) and has been let go in this world to choose and find God ; so when he returns back; only God can judge how well it perform based on context/circumstances and its abilities and since God is just , wise, all knowing it will judge based on truth ( there is no garauntee if you are Muslim you will go x or y)

Also Islam claims that all religions are monotheist religions; before its followers corrupted it and add and subtracted things ( so current Christianity we know is not the original that Jesus brought ; any ways that is a separate conversation)

Wrote a piece how monotheism will help solve lot of our issues and set this world to balance which we are missing on

https://medium.com/illumination/monotheism-the-religion-of-all-prophets-5718c20ad0c1