The POTTER’S clay in scripture REPRESENT God’s people His church in its purity.
“But now, O LORD, thou art our father; we are the clay, and thou our potter; and we all are the work of thy hand.” “O house of Israel, cannot I do with you as this potter? saith the LORD. Behold, as the clay is in the potter’s hand, so are ye in mine hand, O house of Israel.”
MIRY CLAY is unstable USELESS, MUCK, mud.
Psalm 40:2 (KJV) He brought me up also out of an horrible pit, out of the miry clay, and set my feet upon a rock, and established my goings.
When Christianity is contaminated with popery you get useless miry clay in the end of time.