What is the purpose of Satan?
Satan was not created as a devil--he was a morning star who was in harmony with God. He became Satan (Adversary, accuser) when he allowed sin to enter his heart.
He is allowed to continue for a time to educate mankind about sin and its affects. Paul calls this the "present evil world"--but also talks about another one where in dwells righteousness.
The whole purpose of God allowing sin to infect all of humanity is for our education. As with any trial, God promises a way of escape. And in the Gospel Age that way of escape is Jesus.
All in Adam die--but all in Christ shall be made alive. Jesus died for Adam--and all who were in Adam--that is us. Jesus didn't die for "all sin", but for the sin that Adam committed and infected everyone else with. By eliminating that sin, when we come into Christ we are freed from Adamic Sin and the curse and are "New Creatures" in Christ.
The purpose of the Gospel Age is the calling of the Bride of Christ---it not not a general calling of the world--else what do we do with the millions upon millions who died before Jesus opened the New and Living way?
Once the Bride is complete as shown in the parable of the wheat and the tares, wise and foolish virgins and many other places--then God will deal with the world of mankind. Judgment begins at the house of God.
Those who prove to be faithful unto death will then be Kings and Priest in the Kingdom we all pray for. Then the resurrection of the dead of all humanity all the way back to Adam will take place. "All who are in the grave will hear the voice of the son of God and come forth."
Purpose of the King is to rule and judge, the purpose of the priest is to reconcile back to God. "When the judgments of God are in the world the world will learn righteousness." "All will know the Lord from the least unto the greatest" At the end of the Millennium God will be All in All.