Contradiction with John 9:3
This verse suggests that the man's blindness was not due to sin, whereas John 9:21 alludes to the parents' uncertainty about their son's condition.
John 9:3: Jesus answered, Neither hath this man sinned, nor his parents: but that the works of God should be made manifest in him.
Contradiction with John 9:34
This verse accuses the man of being entirely born in sins, contrasting with John 9:21 where his parents avoid confirming any sin-related cause.
John 9:34: They answered and said unto him, Thou wast altogether born in sins, and dost thou teach us? And they cast him out. [cast...: or, excommunicated him]
Contradiction with Exodus 20:5
This verse speaks of visiting the iniquity of the fathers upon the children, contradicting the parent's claim of ignorance about their son's blindness in John 9:21.
Exodus 20:5: Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God [am] a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth [generation] of them that hate me;
Contradiction with Ezekiel 18:20
Here, it is stated that a son shall not bear the iniquity of the father, opposing any implication in John 9:21 that the man's blindness might have been inherited.
Ezekiel 18:20: The soul that sinneth, it shall die. The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son: the righteousness of the righteous shall be upon him, and the wickedness of the wicked shall be upon him.
Contradiction with John 9:2
Here, the disciples directly ask if the man's blindness is due to sin, suggesting a belief in sin-related causes contrary to the ambiguity in John 9:21.
John 9:2: And his disciples asked him, saying, Master, who did sin, this man, or his parents, that he was born blind?