In ancient times, rich people sometimes kept their treasures in simple clay jars. Paul probably thought of this when using this metaphor.
(2) 2 Timothy 2:20-21: “In a large house there are articles not only of gold and silver, but also of wood and clay; some are for special purposes and some for common use. Those who cleanse themselves from the latter will be instruments for special purposes, made holy, useful to the Master and prepared to do any good work.”
Ephesians 2:10: “For we are God’s handiwork, created in Christ Jesus to do good works, which God prepared in advance for us to do.”