Rev.Russell Moore's Bible Study:

A clean vessel is a useful vessel to God. The scriptures commands all believers to have on the breastplate of God's imputed righteousnes at all times. King David prayed after he has sinned, create in me a clean heart. The imputed holiness of Jesus must be met by a practical holiness worked out by you and I.  Your shield and Sword but not become rusty, have weak spots, the devil looks for those places to attack you. You must daily work out your salvation.