"I promise I'll do it. I swear!" How many times have we said something of this nature? We think that somehow a phrase like this increases the chances of us keeping our word. And yet, Jesus calls His followers to a different standard of truth-telling.
As He has done three times already, Jesus addresses a cultural standard of the day (swearing by oaths), and begins to reshape how His followers are to act. Much like our own day, someone's word was seen as somehow more true if they took an oath. This was because there was a lackadaisical attitude when it came to keeping your "regular word." And it is very similar in our day-and-age. It almost seems like there is a "sliding scale" of truth-keeping, from flat-out lies to swearing by an oath, with other degrees of truth-telling in-between.
It seems that the Pharisees and scribes thought that God only noticed when they swore by something (such as heaven or earth or Jerusalem or their own head). In reality, Jesus makes it clear that God sees us and the words we speak, all of the time! He calls us to keep our word as His followers. Therefore, when we say "yes," we should mean it. And when we say "no," we should mean it. Christians should be people of integrity.
Now, this does not mean we are not to swear in court, or in other settings like that. What Jesus was referencing was a problem of only being honest when it seemed necessary. Jesus was addressing a specific issue, one which we still have today.
And of course, all of this is rooted in Christ. He is the ultimate model of what it means to live by integrity. As 1 Peter 2:22 says of Jesus, "He committed no sin, neither was deceit found in his mouth." None of us can perfectly live to that standard, which is why I am so thankful for the Gospel. And yet, in response to His grace, we are called to follow in His truth-telling footsteps. May we all, more and more, become people of integrity!