That Which Defiles

In context, when Jesus says that nothing going into a person's mouth makes them unclean, he is not speaking of eating food prohibited in the dietary laws of Leviticus 11.  He does not mention or even hint at the eating of pork, shark, or crawfish either as a suddenly permissible thing or as something he is touching upon at all.  No, he specifies the exact action he is focusing on early in Matthew 15, when some of the Pharisees complain that his disciples do not wash their hands before eating (15:1-3).

The Pharisees that he says invent asinine traditions supposedly in service to God (in violation of the command of Deuteronomy 4:2 to not add to obligations, as if doing what is good is not enough to truly be righteous) set aside the actual obligations rooted in Yahweh's nature, Jesus points out, such as the obligation to execute those who curse their mothers or fathers (Exodus 21:17, Matthew 15:4).  Instead, they submit to social constructs and personal preferences, all of which are meaningless and irrelevant to the truth.  Washing hands prior to meals is not prescribed in the Torah.

What goes into someone's mouth, even potential dirt, does not defile them, but what comes out of their mouth does, according to Jesus: the context is simply that of him affirming Mosaic Law as just even with its executions, so he is not denying or in any way contradicting the core tenets of the Torah.  Though they are far less central and important than the criminal justice laws or those clarifying how not to act towards God, the dietary laws are among God's commands.  Jesus does not oppose them or say that God's nature has changed regarding this issue (Malachi 3:6).

There is nothing about the dietary laws tied to situational circumstances like the Temple, without which there can be no sin in not offering sacrifices at the Temple, for the building is not standing.  One cannot be obligated to do something one cannot do, and one can go a lifetime eating that which is permitted in Mosaic Law--and there are plenty of foods, from meats to vegetables to fruits to even bugs, that are allowed.  For those who have been eating the prohibited foods on a regular basis, it might be very inconvenient to narrow or shift their diet, but it is not impossible.

Arbitrary customs like praying before meals or always washing hands before eating would be the real morally unnecessary deeds.  One could do certain needless things like these if one wished to, given that they do not violate real obligations by default.  Since they are not obligations, and even if they were, making favorable assumptions about them would be irrational (and any assumptions), and looking to irrelevant factors like conscience or social norms instead of morality is irrational and immoral.  The Pharisees added to Mosaic Law while actively failing to practice it.  This was one of their errors in the story of Matthew 15, and the dietary laws are not even slightly challenged by Jesus here.