This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong; it cannot perjure itself; it cannot be wholly absent. Only its interpretation can err.
Oath definition, a solemn appeal to a deity, or to some revered person or thing, to witness one's determination to speak the truth, to keep a promise, etc.: to testify upon oath. See more. Dictionary.com
A loyalty oath is an oath of loyalty to an organization, institution, or state of which an individual is a member. In this context, a loyalty oath is distinct from a pledge or oath of allegiance. It is an affirmation by which a person signs a legally binding document or warrant. 1 In the United States.