Contradiction with Matthew 6:6
Contradicts public prayer or being seen by others while praying, whereas Luke 9:18 involves Jesus praying with disciples nearby.
Matthew 6:6: But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.
Contradiction with John 18:20
Jesus claims to have spoken openly to the world, contrasting the private nature of the prayer in Luke 9:18.
John 18:20: Jesus answered him, I spake openly to the world; I ever taught in the synagogue, and in the temple, whither the Jews always resort; and in secret have I said nothing.
Contradiction with Matthew 16:21-23
Peter's rebuke of Jesus about his suffering contrasts with the acknowledgment of Jesus' messianic identity in Luke 9:18 without dispute.
Matthew 16:21-23: From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.
Contradiction with Mark 8:27-33
Similar to Matthew 16:21-23, highlights Peter's misunderstanding despite the private prayer setting of Luke 9:18.
Mark 8:27-33: And they answered, John the Baptist: but some [say], Elias; and others, One of the prophets.
Contradiction with Genesis 32:24-30
Jacob wrestles alone, emphasizing solitary prayer as opposed to Jesus praying with others nearby in Luke 9:18.
Genesis 32:24-30: And Jacob was left alone; and there wrestled a man with him until the breaking of the day. [breaking...: Heb. ascending of the morning]
Contradiction with Isaiah 29:13
Critiques honoring God with lips only, which contrasts with the personal, direct engagement in prayer seen in Luke 9:18.
Isaiah 29:13: Wherefore the Lord said, Forasmuch as this people draw near [me] with their mouth, and with their lips do honour me, but have removed their heart far from me, and their fear toward me is taught by the precept of men: