But above all, my brethren, do not swear, either by heaven or by earth or with any other oath. But let your "Yes" be "Yes," and your "No," "No," lest you fall into judgement. – James 5.12
It is apparently really important that we be people of our word. Here James repeats the Sermon on the Mount principle about not taking oaths or swearing.
I had just studied through Jesus’ words a few weeks ago when I was called for jury duty. When it came time to take the oath I was a bit anxious because I wanted to do the right thing. I don’t really think there is a problem with taking a legal oath in court, but I just wanted to do right. So when it came to my turn I asked for an affirmation where I just declared that I would do what was required of me.
That is probably taking it a bit too far, but there is a principle of Christians being truth-tellers.
We ought never to have to swear to the truth in order to be believed. I should never have to ‘promise to do it’ or ‘swear on a stack of Bibles’ or anything of the sort. If we say it everyone need to know that we are going to do our best or do our very best to do it. We should not let people down unless God’s changes our plans.
Things are going to come up. We don’t know what tomorrow holds. All of our plans should have a ‘D.V.’ attached – but if we are not people of our word how are others going to have any faith in us when we share the gospel?