The Loyalty Oath aka Ironclad Oath was a retroactive instrument used to oppress ex-Confederates. That is why my ancestor thought it was un-Constitutional.

The Ironclad Oath was a key factor in the removing of ex-Confederates from the political arena during the Reconstruction of the United States
It required every white male to swear he had never borne arms against the Union or supported the Confederacy — that is, he had "never voluntarily borne arms against the United States," had "voluntarily" given "no aid, countenance, counsel or encouragement" to persons in rebellion and had exercised or attempted to exercise the functions of no office under the Confederacy. Its unpopularity among ex-Confederates led them to nickname the oath "The Damnesty Oath."