So, an omnicient God benevolence is so great that he lets some kind of evil figurehead tempts his supposedly beloved children into sin.
1. Depends if Satan is indeed evil, or if he's an agent of God, depending on some beliefs.
2. Free will. God wouldn't be respecting free will if he didn't let evil have its hand too, since he wouldn't be giving us the real power to choose. You may as well not have the ability to decide what to eat if you're only given the same thing for each meal.
3. A test of faith. By allowing an evil being to tempt us towards sin and evil, God is testing each individual person's faith and loyalty to him. Indeed, I wouldn't be surprised if some crafty parents ask someone to tempt their kid away from them on occasion, to test the child's respect for their authority.
And the rest is just up to part - omniscient God that isn't omniscient
Well, the future would be predetermined if God knew the exact future. Him knowing all the possible futures is more in line with the idea that the future isn't written, but is instead something that could easily change, given that it is the combined actions of all life on Earth. As an extension, this concept of his omniscience is more in line with free will.
The past has already happened, so God would know that without complication; the present already is, so likewise. But the future has not, and so God knows only the possible futures, and probably would try to steer us in the direction of the favorable futures. ...Of course, we are free to disregard his advice.
perfect being creating/letting create evil, and so on.
God is all about choice. Therefore, he must allow evil to exist, or otherwise, he's not giving us choices. Just as he lets humans believe in him or reject him.
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And I'm saying this all as a non-religious person.