“Congress defined citizenship.”
Congress has Constitutional authority to define the naturalization process. The Constitution, through the 14th Amendment, defines a citizen of the US.
A Natural born citizen must be native born and cannot be required to swear an Oath of Allegiance because allegiance is a natural or implied without reservation. Not all native born persons are Natural born citizens because some are born with dual allegiance or their allegiance to the US requires an affirmation before doubts are resolved.
Obama’s campaign made a big deal of Obama’s Kenyan citizenship obtained in 1963 expiring because he did nothing about it. In fact, Obama should have affirmatively renounced his Kenyan citizenship at the age of majority to secure his allegiance to the US.
How can you define “Naturalization” if you are not sure of who NEEDS a Naturalization process and who does not need such a process, due to automatic Citizenship?
Besides, no Court case has ever challenged the right of Congress to define Citizenship, and Congress also has the power to interpret and define the 14th Amendment.