From John Piper’s A Godward Life on practical steps to kill sin:

Consciously reckon the old man dead; that is, believe the truth of Scripture about the old man’s death in Christ and seek to live in that freedom (Romans 6:11). Living out the reality that you are is the proof that you are. One clear illustration of becoming what you are is found in 1 Corinthians 5:7 (RSV): “Clean out the old leaven that you may be a new lump, as you are unleavened.” It sounds strange, but salvation is a strange and wonderful thing: Clean out the old leaven of sin, because it is really already cleaned out. If you try to play logic games with this reality and say, “I don’t need to fight sin because it is already cleaned out,” you will prove only that you are not among the number who are cleansed.