If there wasn't an opponent, an opposing side, God's people would never be tried and tested. So He allows Satan to rise up and lead the opposition, to try and test and persecute His people.
Persecution is to test the faith of the followers, to see who's really got it and who hasn't. These tests have to come to test their faith and their determination and loyalty. Persecution makes the weak weaker, but the strong stronger, and only the best survive.
It's a part of God's sifting and winnowing process to separate the wheat from the chaff, the good grain from the tares or weeds, the sheep from the goats. For His true sheep know His voice and they follow Him, and a stranger they will not follow. (Jn.10:4,5) They will not listen to and follow the voice of the enemies of the Truth and their criticisms and condemnations and their scandalous lies. The true grain will last and the true sheep will follow.
Those who are not willing to suffer and endure persecution for their faith are purged from the ranks, but the genuine faithful followers call on God and God always ultimately defeats the Devil and wins the victory!