Before entering upon the duties of their respective offices, the said commissioners shall take and subscribe before the judge of the court of record having jurisdiction of the trial of criminal cases in the county in which the respective cities for which they are appointed to serve shall be located, or the clerk thereof in vacation, the oath or affirmation prescribed by the Constitution of the state of Missouri, and shall also take and subscribe before the same judge, or clerk in vacation, the further oath or affidavit that they will in no case and under no pretext act on behalf of or against, prefer or disfavor, any policeman or officer of police or other employee of the board for or on account of the political opinions of such person, or for any other cause or reason whatsoever than the fitness or unfitness, merit or lack of merit of such person, in the best judgment of such commissioners, and that during their tenure as members of the board of police commissioners (other than the mayor) they will refrain from partisan political activity, violation of which shall be sufficient cause for removal of such members of the board. The said oaths or affirmations shall be recorded and preserved among the records of said court.