Affirmation in law
In law, an affirmation is a solemn declaration allowed to those who conscientiously object to taking an oath. An affirmation has exactly the same legal effect as an oath, but is usually taken to avoid the religious implications of an oath; it is thus legally binding but not considered a religious oath. Some religious minorities hold beliefs that allow them to make legally binding promises, but forbid them to swear an oath before God. In some jurisdictions, an affirmation may only be given if such a reason is provided.
United Kingdom 
A right to give an affirmation has existed in English law since 1695, when An Act that the Solemne Affirmation & Declaration of the People called Quakers shall be accepted instead of an Oath in the usual Forme (7 & 8 Will. 3 c. 34) was passed. The text of the affirmation was the following: "I A.B. do declare in the Presence of Almighty God the Witnesse of the Truth of what I say.". The right to give an affirmation is now embodied in the Oaths Act 1978, c.19, which prescribes the following form: "I, do solemnly, sincerely and truly declare and affirm," and then proceed with the words of the oath prescribed by law, omitting any words of imprecation or calling to witness.
It has its origins in the refusal of Quakers to swear any oath, which would otherwise have barred them from many public positions. Quakers believe in speaking the truth at all times, and so the act of only swearing to truth in court, rather than in everyday life would imply double standards. As in James 5:12, they tried to "let your yea be yea and your nay be nay".
The cause for such a right is best shown in cases such as R v William Brayn (1678). Here, William Brayn was charged with the theft of a horse from Quaker Ambros Galloway. Brayn pleaded 'not guilty'. One witness testified that the horse was owned by Ambros Galloway; and another witness said that he [probably Galloway] bought it from Brayn. As Galloway was a Quaker, he would not, "for conscience-sake", swear and so could give no testimony. The Court directed the jury to find Brayn 'not guilty' for want of evidence, and committed the Quaker, "as a concealer of Felony", for "refusing an Oath to Witness for the King".
Some Christians, who may not be Quakers, refuse to swear oaths, based on Matthew 5:34-5:37.
United States of America 
The original 1787 text of the Constitution of the United States makes three references to an "oath or affirmation": In Article I, Senators must take a special oath or affirmation for the purpose of sitting as the tribunal for impeachment; in Article II, the president is required to take a specified oath or affirmation before entering office (see oath of office); and in Article VI, all state and federal officials must take an oath or affirmation to support the U.S. Constitution. A fourth appears in Amendment IV, all warrants must be supported by evidence given under oath or affirmation.
See also 
- Statutes of the Realm: Volume 7. 1820 British History Online, the Institute of Historical Research and the History of Parliament Trust. Accessed 19 February 2008)
- Official text of the Oaths Act 1978 as in force today (including any amendments) within the United Kingdom, from the UK Statute Law Database
- "William Brayn, Theft > animal theft, 11th December 1678.". Old Bailey Proceedings Online. 1678-09-06. Retrieved 2008-04-27. Reference Number: t16781211e-37