The presence of an evil world system provides a choice to man, something to choose Christ against, or not choose Christ against. It also eliminates false believers,
Matthew 13:20-22 But he that received the seed into stony places, the same is he that heareth the word, and anon with joy receiveth it; Yet hath he not root in himself, but dureth for a while: for when tribulation or persecution ariseth because of the word, by and by he is offended. He also that received seed among the thorns is he that heareth the word; and the care of this world, and the deceitfulness of riches, choke the word, and he becometh unfruitful.
Evil has been used to discipline believers,
2 Corinthians 12:7 And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure.
1 Timothy 1:19-20 Holding faith, and a good conscience; which some having put away concerning faith have made shipwreck: Of whom is Hymenaeus and Alexander; whom I have delivered unto Satan, that they may learn not to blaspheme.
Evil spirits and people may not be up to good, but God can make good of their evil,
Genesis 50:20 But as for you, ye thought evil against me; but God meant it unto good, to bring to pass, as it is this day, to save much people alive.
Especially with regard to sincerity, and given we have free will, I don't know how God would sort the "boys from the men" in a faith sense, if the Christian had no opposition, if there wasn't a cost involved. If it were too easy, how many lip service, "fair weather friends" of God would there be, who'd never been tried, unfit for the kingdom? The cross, itself, was a cruel evil, an instrument of torture murder, yet,
Matthew 10:38 And he that taketh not his cross, and followeth after me, is not worthy of me.
In Bible history, and I fear in our world, now, God uses evil nations and leaders to punish the reprobate nation, while God not the author of the evil, it is used to accomplish His pleasure, at the hands of those who care nothing for the pleasure of God. We have examples of wayward Israel being taken captive back when, and, in our times, things like the 2016 election.
I think it's much to God's glory, attests to the triumph of good over evil, that He can take the very schemes of Satan and turn them on their head, for good, despite the whole environment being one of evil. We learn a lot from evil's weaknesses and failures.
Again, you need something to choose against, to have a choice, in the first place,
Joshua 24:15 And if it seem evil unto you to serve the LORD, choose you this day whom ye will serve; whether the gods which your fathers served that were on the other side of the flood, or the gods of the Amorites, in whose land ye dwell: but as for me and my house, we will serve the LORD.