You ask “If we deem it lawful for a witness in a court of law to take the oath usually taken by witnesses,”
This is entirely a question for individual conscience. “Let every man be fully persuaded in his own mind.” We deem it perfectly right for a witness, when called upon by the magistrate, to take the oath usually administered. Under the Mosaic economy, when anyone heard “the voice of adjuration,” he was bound to utter, or give evidence. (See Lev. 5) And when the High Priest said to our blessed Lord, “I adjure thee by the living God,” He immediately spoke. The magistrate is the expression of the power of God, and when he adjures us, we are bound to give evidence. But, as we have said, let each one act according to his light. The enlightened and liberal government under which we, in England, are privileged to live, has, with characteristic leniency and considerateness, made provision for sensitive consciences in an Act of Parliament to which you refer. The form which such persons must use commences thus: “I, A. B., do solemnly, sincerely, and truly, affirm and declare, that the taking of any oath is, according to my religious belief, unlawful.” Nothing can more fully illustrate the genius of the British Government, than the wording of this form, May God, in his infinite mercy, bless our Queen and preserve our constitution! May His gracious providence strengthen the pillars of that throne, under the shadow of which we enjoy a measure of freedom hardly known by the subjects of any other Government in the world.