From the Ecclesiastical Dictionary 1900 page 502
"An oath is a solemn affirmation in which we invoke the name of God, tacitly or explicitly, as witness to the truth of a statement.
An oath is permissible in justice and in truth, when circumstances are of sufficient importance.
An oath should be taken "in truth, and in judgement, and in justice" (Jer.iv.2)" that is to say, affirming with adequate motive a thing of which we are morally certain or promising what we actually mean to perform.
Without these three conditions of integrity, namely, a solemn affirmation or promise, importance of matter, and equity of motive, an oath would unquestionably be disrespectful to God, and must therefore be a mortal or venial sin, according to the gravity of the circumstances, or intention and opinion of the person taking the oath"
Now if we have separation of church and state, why would the state require us to call upon Almighty God to witness what we are about to speak? They've kicked Him out of state affairs so why should we put our souls in peril more severely by making an oath?
In centuries past there was a large crucifix behind the judge, and when a man was condemned the judge said,"May God have mercy on your soul" TIMES HAVE CHANGED.
A Religious vow is totally different than an oath taken in court which is what this thread is about.