Ephesians 3:10. “To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God.”
The great mystery, the Church, was not revealed to anyone–not even the hosts in heaven–until Jesus Christ first proclaimed it in Matthew 16:18. It was not revealed in the Old Testament, but was hidden in the mind of God until the time was right. Then, through Jesus Christ, the mystery was revealed even to the angelic hosts of heaven, that God, in His incredible wisdom, was establishing a new thing in the Church, where all believers would come together as one body to proclaim the gospel of the grace of God to all mankind.
Of course, Satan had a toe-hold in that process right from the beginning. He has been busy creating dissension, hatred, anger, and division ever since.
However, Jesus said that the gates of hell would not prevail against His church, and that is the promise we must hold onto.