What does Matthew 5:35 mean?
Jesus is showing how the shallow teaching of Israel's religious leaders misses the intent of God's Word. He has introduced statements using the phrase "you have heard…" and followed that with "but I say…" in order to give a fuller explanation.One example of how shallow legalism wound up contradicting God's intent was in the category of oaths. Israel's leaders had apparently tolerated a system of promises between Israelites for various contractual agreements. Swearing by the Lord was considered a solemn thing, binding the oath-taker to follow through or face God's judgment (Numbers 30:2). Swearing by heaven, earth, or other things, though, was allowed as a less serious and perhaps less binding form of agreement. Unfortunately, many of those oaths provided room for deliberate deception.
The modern equivalent to this is the casual oath, which in English is often started with the phrase "I swear…" Using some additional promise or guarantee to earn trust is what Jesus has in mind here. His target is not official vows such as in courtrooms or legal contracts.
Jesus doesn't specify certain oaths as good and some as bad. Rather, He discards the value of casual oaths, entirely (Matthew 5:34). For one thing, swearing by something that belongs to God comes very close to swearing by the Lord Himself. So, Jesus has said not to swear by heaven because it is God's throne. Now He adds that they must not swear by earth because it belongs to God as His footstool, meaning that He can do anything He wants with it.
Also, Jesus adds, don't swear by Jerusalem—or possibly "towards" Jerusalem—because it belongs to the Great King. This may be another name for God. If so, swearing by or towards Jerusalem also comes dangerously close to swearing by the Lord.
The overall point is that a Christian's life should reflect a pattern of truth and integrity. When a born-again believer says "yes" or "no," they should mean it with complete honesty (Matthew 5:37). In such a case, there's nothing to be gained from adding oaths on top of one's word.