Ephesians 6:10-12 "A final word: Be strong in the Lord and in his mighty power. Put on all of God’s armor so that you will be able to stand firm against all strategies of the devil. For we are not fighting against flesh-and-blood enemies, but against evil rulers and authorities of the unseen world, against mighty powers in this dark world, and against evil spirits in the heavenly places."
I heard someone say one of Satan's greatest achievements is convincing people that he doesn't actually exist. I would say Satan's the second great achievement is infiltrating the Church, and convincing people that the Gospel message passed down through the ages is outdated and in need of change.
On that note, I found this article in Christianity Today very interesting.