Matthew 5:33,34

"Again, you have heard that it was said to them of old time, Thou shalt not on-oath, but shalt from-give unto the Affirmer thy oaths: 34 but I say unto you, swear not at all..."

In both the Old Testament and the New, the importance of doing what we promise is critically important.  But, there's a definite difference in how we personally assure others (and God) that we will perform as stated. In the verse and a half above, Jesus makes this difference as clear as possible.  He says, in the Etymological New Testament that "Thou shalt not on-oath..."  The ENT's ultra-literal translation allows the reader to see the two-part word Jesus used to indicate a superficial ("on") oath.  The OT forbade superficial oaths, on-oaths.  One had to genuinely "from-give" what one had sworn.  This is one method to help ensure that one does as promised: make the oath more elaborate.  Add a phrase like "so help me God" to a promise.  Perhaps, if we make our oaths more intense and complex, we will really perform them. 

Jesus takes a different tact.  By stripping our affirmations of all verbal clutter, we are left with simply our Yes or our No.  Nothing else...except our integrity.  This minimalist approach is the New Testament answer to human perfidy. We do as we say because we wish to retain our dignity, our self respect.  We claim allegiance to Jesus and his teaching.  And now we will do as we have promised.