Article 2 Section 1 of the Constitution reads: No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty-five Years, and been fourteen Years a Resident within the United States.
If two citizen parents were not required for ‘Natural Born Citizen’ status then there is no possible logical reason for the adoption clause in Article 2 Section 1. All the founders were charter citizens with non-citizen parents because the country did not exist when their parents gave birth to them. And there were many citizens at that time who were born on U.S. soil but not to citizen parents for the same obvious reason.  The only distinguishable difference between the “citizens at the time” and a “natural born citizen” is the citizenship of the parents at the time of birth. The founders knew it would take a generation to produce the first ‘Natural Born Citizen’ born on U.S. soil from parents who were citizens to produce a candidate free from any direct foreign birthright allegiances. The founders needed to include the charter citizens in order to have Presidential candidates (themselves) until a ‘Natural Born Citizen’ could be available for candidacy. At the time of the adoption there were only two groups of charter citizens available for the candidate pool…native born citizens (born on U.S. soil to non-citizen parents) and naturalized citizens (those born abroad). If either of these groups were eligible to hold office as President then there would be no reason for the adoption clause nor would there be a need to distinguish ‘Natural Born Citizens’. And to those who would suggest that the adoption clause was because the ‘soil’ was British before the adoption and that it was strictly a matter of jus soli, the article would read ‘no person except a native born citizen’ instead of ‘no person except a natural born citizen’ as it was well understood and a part of the language of the day to regard a person born on the soil a native born citizen.
President Chester Arthur was born on US soil to a US citizen mother, but faced an eligibility challenge because his father was not a citizen when Chester was born. Before the authorities could seize them Arthur took all his family documents and burned them in his back yard effectively covering up the fact of his ineligibility to hold office. It was only recently in 2009 that a researcher found documentation which confirmed that Arthur’s critics were right about his ineligibility because his father was not a citizen at the time of Chester’s birth.