Omniscience is all-knowledge. John says that "God is greater than our heart and knows all things.” (1 John 3:20, emphasis mine) God showed this to Job when He took him all over the world and revealed the details no one else could know (see Job 38-39). The devil may have showed Jesus "all the kingdoms of the world" (Matthew 4:8) during His time of temptation in the wilderness but his motive was to get Jesus to disobey His Father. And Jesus loves His Father too much to do that. All that knowledge, all that power and influence—while it was rightfully God's—had been stolen and misappropriated by Satan. The devil doesn't have all knowledge. We see this as he couldn't even tell that Job was vulnerable (see Job 1:10). With Lucifer (Satan), we have a perfect example of what Paul calls "Ever learning, and never able to come to the knowledge of the truth.” (2 Timothy 3:7) The knowledge that you are loved by God. That is truth. That is perfection. When we forget that God is love, we end up on a quest for knowledge when what we need is love. "To know the love of Christ which passes knowledge" (Ephesians 3:19).
That which caused the devil to go crazy with conquest—knowledge and power—humbled Job. And while Job came again to know the love of God, the worldwide influence that Satan had was stripped when Jesus defeated him in death. And resurrection. When the devil tempted Jesus by revealing all the kingdoms of the world and offering them to Him in exchange for acquiescence and obeisance, it should be understood that he couldn't offer what he didn't have to give.
Knowledge might be power. But without influence, what good is it?
The term "know-it-all" has always had a negative connotation. So what good is the acquisition of knowledge for its own sake? Colossians (2:3) says that "in whom (Jesus) are hid all the treasures of wisdom and knowledge." It goes on to say, in sewing up such a bold statement, that we “are complete in Him who is the head of all principality and power". Those two things principality and power are what (along with Satan, himself) Jesus encountered when the devil tempted Him in the desert and then subsequently conquered when He died and rose again. Jesus said that the hairs on our head were numbered (see Matthew 10:30). So are the stars (see Psalm 147:4). But what is that without love? Paul says in Corinthians that even if he had "all knowledge", bereft of love, he was "nothing". (1 Corinthians 13:2)
So how does this relate to God's omniscience? I am reminded of a quote from J. I. Packer, who said, very simply that, “God is omniscient, but I am responsible". This means that God isn't obligated to reveal Himself and the depth of what He knows if we aren't humble and love Him to begin with and in return. He doesn't stay where He's not wanted. Does this exonerate God from culpability regarding world disasters and tragedies when they could have been prevented by someone who was omniscient? And omnipotent? Yes. Because God tries to prevent things, by urging His people to pray: "Pray without ceasing" (1 Thessalonians 5:17). A heavy responsibility to be sure, but one without any negative consequences. Many Christians will point to this sin and that sin and the ensuing disasters and tragedies as the logical outcome of such actions. I would have to say that, while sin can and does have dire consequences, Christians who don't realize and appropriate God's help—through His omniscience, omnipresence and omnipotence—certainly have their share in the blame for what goes on in the world. Please forgive us. This is why we need God and His love, in totality.
Deuteronomy 29:29 says that "the secret things belong unto the Lord our God". It continues, but what it doesn't say is that He won't share those "secret things" with us. Have we ever asked God to show us what He knows? Or do we already know it all?