Verse: Matthew 5:33-37
33 Again, you have heard that it was said to the people long ago, 'Do not break your oath, but fulfill to the Lord the vows you have made.'
34 But I tell you, do not swear an oath at all: either by heaven, for it is God's throne;
35 or by the earth, for it is his footstool; or by Jerusalem, for it is the city of the Great King.
36 And do not swear by your head, for you cannot make even one hair white or black.
37 All you need to say is simply 'Yes' or 'No'; anything beyond this comes from the evil one.
In this passage, Jesus continues his pattern of "You have heard it said...but I say...", this time taking on casual oaths. It's important to note that he is not addressing formal instances, such as wedding vows, the Hippocratic oath a doctor would take, or a courtroom witness or political official being sworn in. Rather, he is referring to the addition of a phrase like "I swear" to our casual conversation, in order to emphasize our honesty.
I'd bet we can all think of at least one person in our lives who embodies what Jesus is talking about. The one who always overemphasizes their sincerity, who SWEARS what they're saying is true. And, not coincidentally, they are often the same person whose word we trust the least. Whenever I hear someone say, "I swear, I didn't do it", I get a flashback to high school English class and Shakespeare's Hamlet: "The lady doth protest too much, methinks."
Jesus spends a couple of verses explaining why it's not a good idea to swear on things. In summary, the Jewish culture thought swearing on God would bind the speaker to fulfill their oath, but Jesus points out that since everything belongs to God, swearing on anything is essentially swearing on God. In verse 37, he gives us the bottom line; if we are people of integrity, if we are known to be trustworthy, if we are men and women of our word, those are attributes that can't be improved upon by adding a phrase like "I swear" to what we say. Rather, people will know that when we say "yes" or "no", that is exactly what we mean, and we can be trusted to follow through.