Contradiction with John 14:13
Contradicts by assuring that whatsoever is asked in Jesus' name will be done, contrasting with the disciples’ failure in Luke 9:40.
John 14:13: And whatsoever ye shall ask in my name, that will I do, that the Father may be glorified in the Son.
Contradiction with Matthew 17:19
Disciples question their lack of ability, highlighting their failure in Luke 9:40.
Matthew 17:19: Then came the disciples to Jesus apart, and said, Why could not we cast him out?
Contradiction with Mark 9:18
Similar account, yet adds more context on the disciples' inability, contradicting their expected power.
Mark 9:18: And wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away: and I spake to thy disciples that they should cast him out; and they could not. [teareth...: or, dasheth him]
Contradiction with Matthew 21:22
Promises that believing and prayer will lead to receiving, conflicting with the disciples’ inefficacy in Luke 9:40.
Matthew 21:22: And all things, whatsoever ye shall ask in prayer, believing, ye shall receive.
Contradiction with Matthew 10:1
Disciples were given authority over demons, contrasting with their failure to cast out the spirit in Luke 9:40.
Matthew 10:1: And when he had called unto [him] his twelve disciples, he gave them power [against] unclean spirits, to cast them out, and to heal all manner of sickness and all manner of disease. [against: or, over]
Contradiction with Mark 11:24
States that whatever is asked in prayer will be granted if believed, differing from the disciples' ineffective attempt in Luke 9:40.
Mark 11:24: Therefore I say unto you, What things soever ye desire, when ye pray, believe that ye receive [them], and ye shall have [them].
Contradiction with Matthew 17:21
Indicates that some actions, like casting out this kind of demon, requires prayer and fasting, suggesting reasons for failure in Luke 9:40.
Matthew 17:21: Howbeit this kind goeth not out but by prayer and fasting.