Contradiction with Matthew 13:31
This verse uses a similar parable to describe the kingdom of heaven rather than the kingdom of God, which might create confusion about whether they are identical or distinct concepts.
Matthew 13:31: Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field:
Contradiction with John 18:36
Here, Jesus states that His kingdom is not of this world, which contrasts with Luke 13:18 where the kingdom of God is likened to earthly elements like a mustard seed.
John 18:36: Jesus answered, My kingdom is not of this world: if my kingdom were of this world, then would my servants fight, that I should not be delivered to the Jews: but now is my kingdom not from hence.
Contradiction with 1 Corinthians 4:20
This verse states that the kingdom of God is not in word but in power, which contrasts with Luke 13:18 where a simple parable is used to describe it, possibly minimizing its powerful implications.
1 Corinthians 4:20: For the kingdom of God [is] not in word, but in power.