Swear to tell the truth 'so help me God'? I know that in court if you don't believe in God you are allowed to simply give an "Affirmation" instead of an Oath. But what I want to know is why any Christian would swear an Oath to tell the truth "So help me God"? Where did the idea that this is OK come from? I think the words of Christ were pretty clear, but as far as I know only a few denominations such as Quakers and Mennonites agree with that position (and I certainly don't see eye to eye with them on many key issues). Has there been any justification presented for doing this, or are people just doing what is expected of them from the government?  Here is a relevant passage on the subject: Matthew 5:33-37


  33 “Again, you have heard that it was said to the people long ago, ‘Do not break your oath, but keep the oaths you have made to the Lord.’ 34 But I tell you, Do not swear 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 Simply let your ‘Yes’ be ‘Yes,’ and your ‘No,’ ‘No’; anything beyond this comes from the evil one.
My understanding is that the passage quoted does not forbid all oaths in general, but in specific was addressing the practice of the Scribes and Pharisees, who were invoking the name of God, or heaven in their oaths, and breaking them.  I don't see myself where the Scribes or Pharisees are mentioned specifically.  

I'd have to study it out further, but I do believe that he was teaching based on a general lack of reverence for the name of God.  Taking an oath made in God's name is serious, which is why Jesus warned us to let our yes be yes and our no be no.  It is better not to swear on the name of God than to swear and break it.

There's an excellent article on this here: http://www.reformed.org/webfiles/antithesis/index.html?mainframe=/webfiles/antithesis/v1n1/ant_v1n1_oaths.html

In it, the author explains how and why this applied to the Scribes and Pharisees, not to all oaths in general.

It goes on to cite scriptures in the bible where we are expected, and even commanded to swear oaths in the name of God in certain occasions.

The following is a small excerpt from the articles, where this is explained, with scripture references:


  The general context of Scripture, when studied carefully, reveals that
  God does not forbid all oaths.
  
  First, Scripture commands us to swear by the name of God on certain
  occasions. In Deuteronomy 6:13, for example, Scripture commands God's
  people "You shall fear only the Lord your God; and you shall worship
  Him, and swear by His name." Far from prohibiting all oaths, Isaiah
  tells us that "he who swears in the earth shall swear by the name of
  God" (Is. 65:16). God sanctions lawful oaths to such an extent that He
  promises to build up those who swear by His name (Jer. 12:16). Even in
  the mundane affairs of life, such as confirming the truth between
  disputing neighbors, God commands His people to swear before Him (Ex.
  22:10-11). Because Scripture commands God's people to swear by His
  name, it cannot forbid all oaths. God does not command what He
  simultaneously condemns!
  
  Second, Scripture also teaches us that swearing is an act of
  confession and religious worship. We already saw in Deuteronomy 6:13
  that God commands us to swear in His name precisely because swearing
  in God's name is but one way to worship and fear Him (cf Deut.10:20).
  Isaiah confirms this connection between swearing and worship; when he
  prophesies about the Assyrians and Egyptians coming into a covenantal
  relationship with God, he says that they will swear in the name of
  God. (Is. 19:18). Calvin explains that "by swearing in the Lord's name
  they will profess his religion."[6]
  
  But exactly how is swearing an act of confession and worship? When we
  duly swear in God's name, we confess several things about God. To
  begin with, we confess that God exists. Moreover, we confess several
  of God's attributes as revealed to us in Scripture: we testify that He
  is omnipresent and omniscient, that He is eternal and immutable, that
  He is just and true, that He is powerful and wrathful. By confessing
  His existence and attributes, we also confess that He is the Supreme
  Judge over all the earth and that we are accountable to Him for all
  that we do and say. Though the word of men may fail, the word of God
  never fails. Though men may fail, God never fails. By taking oaths in
  God's name, we confess God to be the ultimate arbiter of truth, and we
  worship the God of truth in spirit and in truth.


Finally, in the context of swearing in in court, or an oath of office, the practice started when more people understood and respected the gravity of swearing such an oath.  It was meant to be binding.  

They believed, correctly, that an oath sworn in God's name should carry more weight, and that it is a serious thing to break it.  Whether or not people still see it that way, or if we're merely repeating words these days, is open to debate, and likely as individual as those taking the oaths.