Contradiction with James 3:8
James 3:8 describes the tongue as an unruly evil, full of deadly poison, suggesting that the tongue can produce harmful words independently of the heart's intentions, potentially contradicting the direct connection between the heart and speech in Matthew 15:18.
James 3:8: But the tongue can no man tame; [it is] an unruly evil, full of deadly poison.
Contradiction with Jeremiah 17:9
Jeremiah 17:9 states that the heart is deceitful above all things and desperately wicked, which can be misinterpreted as suggesting that actions and words emanate from a deceitful heart, still making a case that aligns rather than contradicts Matthew 15:18.
Jeremiah 17:9: The heart [is] deceitful above all [things], and desperately wicked: who can know it?
Paradox #1
The focus of Matthew 15:18 is on what comes from the heart being important. A potential theological conflict could arise when comparing this idea with the emphasis on external rituals and cleanliness in other parts of the Bible, such as Old Testament laws in Leviticus. These laws focus on external actions and rituals, which could seem inconsistent with the idea that internal thoughts and intentions are more significant.
Paradox #2
The potential contradiction in Matthew 15:18 could be that while it emphasizes the importance of inner purity over outer actions, it may conflict with teachings that focus on the significance of actions and behaviors in demonstrating faith and moral character. This can create a tension between what is deemed more important: inner thoughts and intentions or outward deeds and practices.