Persistence in sin destroys the assurance of salvation.
Author: John Piper Source: Jesus Christ is an Advocate for Sinners, Sermon, February 10, 1985
The assurance of the believer is not that God will save him even if he stops believing, but that God will keep him believing--God will sustain you in faith, he will make your hope firm and stable to the end. He will cause you to persevere.
Author: John Piper Source: Sermon: Sustained by the Faithfulness of God, 1 Cor. 1:1-9
It is our privilege to know that we are saved.