Contradiction with Matthew 17:20
Mark 9:18 describes the disciples' inability to cast out a demon, while Matthew 17:20 suggests that with faith as small as a mustard seed, nothing would be impossible, including casting out demons.
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 9:1
Mark 9:18 shows the disciples unable to cast out a demon, whereas Luke 9:1 states that Jesus gave them power and authority over all demons.
Luke 9:1: Then he called his twelve disciples together, and gave them power and authority over all devils, and to cure diseases.
Contradiction with John 14:12
In Mark 9:18, the disciples' failure to perform miracles contradicts John 14:12, where believers are promised they will do works equal to or greater than those of Jesus.
John 14:12: Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater [works] than these shall he do; because I go unto my Father.
Contradiction with Acts 2:4
Mark 9:18 points to the disciples’ failure in casting out a demon, contrasting with Acts 2:4, where they are filled with the Holy Ghost and capable of miraculous deeds.
Acts 2:4: And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.