The Devil and Assurance
Attacks of the Devil are also hindrances to assurance and often have this as their specific aim. Satan knows he cannot ultimately destroy those whom Christ saves. He is therefore determined to destroy our enjoyment of our new relationship to the Lord.
Sinclair Ferguson, The Whole Christ: Legalism, Antinomianism, and Gospel Assurance: Why the Marrow Controversy Still Matters (pp. 219-220)