Contradiction with Matthew 17:20
Unlike Mark 9:20 where a spirit violently convulses a boy, Matthew 17:20 emphasizes faith overcoming obstacles without mentioning such turmoil.
Matthew 17:20: And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.
Contradiction with Luke 9:42
In this passage, the spirit does not explicitly convulse the boy as in Mark 9:20, focusing instead on Jesus' rebuke and healing.
Luke 9:42: And as he was yet a coming, the devil threw him down, and tare [him]. And Jesus rebuked the unclean spirit, and healed the child, and delivered him again to his father.
Contradiction with John 14:27
Mark 9:20 shows violence as part of demon possession, whereas John 14:27 promises peace without reference to spiritual turmoil.
John 14:27: Peace I leave with you, my peace I give unto you: not as the world giveth, give I unto you. Let not your heart be troubled, neither let it be afraid.
Contradiction with 2 Timothy 1:7
This verse states God does not give a spirit of fear, contrasting with the fear and physical distress seen in Mark 9:20.
2 Timothy 1:7: For God hath not given us the spirit of fear; but of power, and of love, and of a sound mind.