Christ and Swearing
Mat 5:33 Again, you have heard that it has been said by the ancients, You shall not swear falsely, but shall fulfill to the Lord your oaths.
In "filling in" the commandments, Christ moves from murder and adultery to trustworthiness. What is interesting about Christ's words here is his choice of words in describing the "old" version of the commandment before he extends it.
Christ doesn't repeat the form of the second commandment Exd 20:7. That commandment in common English translations forbids us "taking the name [of God] in vain." In the original Hebrew, this is the phrase "nasa' shem shav'," which means literally "to lift (or carry) the name falsely." This to me is covers a host of sins and among the worst of them is representing ourselves as doing something for God when we are in fact doing it for some other reason.
Christ comes closer to using the more specific formulation against lying Lev 19:12 of "swearing by the [Lord's] name falsely." In Hebrew, this is "shaba' shem sheqer," which has a cool sound, and means "swear not the name falsely."
The big difference here is that swearing specifically by the name of God isn't mentioned in Christ's version. Christ wanted to make it clear that the issue in the original commandment wasn't using God's name, which has come to take the term "swearing." Instead, he wanted to keep the focus more on honesty itself. The issue here is more clearly on fulfilling our promises. When we make a contract, we must honor it.
In the other versions, we might think the central issue is keeping only the promises that we make to God. By not mentioning God in the first part in connection with making the promise, Jesus tries to make it clear that his focus is on any oaths we make, not just those to God or that involve God. This is consistent with everything else in this section, which focuses on our relationships with one another rather than just our relationship with God alone.
In the last line, the connection is made between what we promise others and what we owe to God. What Christ is saying is that whenever we swear, to whoever we promise, we owe it to God to keep our oaths.