During the Lord Jesus’s Sermon on the Mount, He defined God’s standards for a righteous life. He cited common understandings for principled living and then raised the bar. Among these principles, He declared swearing (i.e., oath taking) off-limits.
“Again you have heard that it was said to those of old, ‘You shall not swear falsely, but shall perform to the Lord what you have sworn.’
But I say to you, do not take an oath at all, either by heaven, for it is the throne of God, or by the earth, for it is his footstool, or by Jerusalem, for it is the city of the great King. And do not take an oath by your head, for you cannot make one hair white or black.
Let what you say be simply ‘Yes’ or ‘No’; anything more than this comes from evil. Matthew 5:33-37 English Standard Version (ESV)
Specifically, Christ prohibits using God’s name (or euphemisms for it) to bolster our sincerity in what we promise or declare to others and to God.
You shall not swear falsely… The man who perjures himself is not the only person who takes the name of God in vain, (Exodus 20:7). He does so, who idly and contemptuously pronounces the name of God on trivial occasions, or in ordinary conversation.
Calvin points out that some have used the Lord’s command to reject all vows. He says they’re mistaken:
Do not take an oath at all… His statement amounts to this, that there are other ways of “taking the name of God in vain,” besides perjury; and, therefore, that we ought to refrain from allowing ourselves the liberty of unnecessary swearing: for, when there are just reasons to demand it, the law not only permits, but expressly commands us to swear.
Stating the obvious, Calvin concludes:
Let what you say be simply ‘Yes’ or ‘No’ Christ now prescribes… that men act towards each other sincerely and honestly… Fairness and honesty in our words are, therefore, demanded by Christ, that there may be no longer any occasion for an oath.
And James, in his letter to the churches, echoed his Lord’s admonition:
But above all, my brothers, do not swear, either by heaven or by earth or by any other oath, but let your “yes” be yes and your “no” be no, so that you may not fall under condemnation. James 5:12 (ESV)
Therefore, let us speak truth, one to another.