Satan the devil deceives the whole world during this 6,000 year age of man (Revelation 12:9). A meaning of "devil" is "slanderer" or "false accuser". He is the accuser of the brethren (Revelation 12:10). But he also accuses and slanders God. He does that by deceiving men with false religion into believing that God is unloving and unjust.
Most mainstream churches teach that to be saved, one must accept Jesus Christ in this life, and that this age is the only opportunity men will have for salvation. Yet the they know that throughout history, hundreds of millions of people never had the opportunity, due to time and chance and circumstances of birth over which they had no control, to accept Christ at any time in their lives, even to the time of their death. Yet according to the theology of this world, they are now condemned, never having been "saved" before they died.
Moreover, much of this same theology teaches that all men have immortal souls, and that those who are unsaved at the time of their death will suffer eternal torment in hellfire.
Talk about a monstrous idea!
But this is Satan's slander against God. He falsely accuses God of being cruel, by torturing men forever in hellfire, and unjust, by condemning those who have never had an opportunity to be saved. He also accuses God of weakness or incompetence, because he accuses Him of being powerless to bring salvation to all mankind.
So through much of this world's religion, Satan falsely accuses God of cruelty, injustice, or weakness, or a combination of these.
Mainstream churches never directly accuse God of these things. They will tell you that God is loving, just, fair, kind, all powerful, all wise, and all good. But those are words they say. They cannot believe those things deep down and still believe the traditional doctrines of their church about the soul, about hell, and about this being the only time of salvation.
The meaning of the Last Great Day shows that Satan's accusations against God are false, and the fulfillment of that day will clear God's name and glorify His reputation.
Ezekiel 37 describes the fulfillment of the Last Great Day. Everyone who has ever lived and never had a chance for salvation will be resurrected back to physical life. At that time, salvation will be open to all, and that includes those among men who have believed Satan's lies. They will find out that God is more loving, more just, wiser, and more powerful than they ever thought before. Those people will rejoice, and God's name will be glorified. People will be able to be re-united with their loved ones, and everyone will have a chance for salvation. I believe most people will choose God's way and enter God's Kingdom.
What a day that will be!
Here are links to related sections in Preaching the Gospel:
The Last Great Day - the White Throne Judgment, Chapter 2