The most important feature of the Act was that it reintroduced a significant new rule of evidence in high treason trials, namely that nobody could be convicted of treason except by the evidence of "two lawful and credible witnesses upon oath... brought in person before him or them face to face," or if he confessed "willingly without violence." This clause of the 1661 Act, section 5, was replaced by the Treason Act 1695, which added that the two witnesses had to have witnessed the same offence. The rule was inherited by the United States and was incorporated into the US Constitution in 1787, which added that both witnesses had to have witnessed the same overt act. Section 5 of the 1661 Act was cited by US Supreme Court justice Antonin Scalia in his judgement in Crawford v. Washington, a case about the "Confrontation Clause" of the Sixth Amendment. The 1661 version of this rule only applied to the new forms of treason which were created by the 1661 Act. The 1695 Act applied to all forms of high treason, except counterfeiting coins. The rule was abolished in the United Kingdom by the Treason Act 1945, which made the rules of evidence and procedure in treason cases the same as in a murder trial. During the passage of the Treason Bill through Parliament the Home Secretary, Sir Donald Somervell, said: Sir William Blackstone wrote in his Commentaries on the Laws of England:
New offences
Treason
The Act created four new kinds of high treason, in addition to those already existing. The Act made it treason to: or, "…within the realm or without, compass, imagine, invent, devise or intend":
to levy war against the King, "within this realm, or without"
to "move or stir" any foreigner to invade England or any other country belonging to the King
These provisions were expressed only to have effect during the lifetime of the King, Charles II. However they were temporarily re-enacted, with two modifications, by the Treason Act 1795, and then made permanent by the Treason Act 1817. In the 1795 version, "the realm" meant Great Britain, and levying war against the King was only an offence under the Act if done in order to compel the King to change his policies or to "intimidate or overawe" Parliament. The difference between the Treason Act 1351 and the Acts of 1661 and 1795 was that while the 1351 Act required an actual levying of war, the later Acts also made it treason to "compass, imagine, invent, devise, or intend" a levying of war. The penalty for treason was death. However the last three treasons on the above list were reduced to felonies by the Treason Felony Act 1848, which made the maximum sentence life imprisonment. The 1848 Act is still in force. Imprisoning or otherwise harming the Sovereign continued to be high treason, punishable by death, until the 1795 Act was repealed by the Crime and Disorder Act 1998. However assaulting the Sovereign is still an offence under the Treason Act 1842, which carries a maximum sentence of seven years. In some Commonwealth of Nations countries, such as Canada, Australia and New Zealand it is still treason to imprison or harm the Queen.
Other offences
Besides treason, the 1661 Act created other offences designed to protect the national security. These offences required the evidence of two witnesses or a confession, and had to be prosecuted within six months of the offence being committed. The king had to personally authorise a prosecution. Anyone who in speech or writing called the King a heretic or papist, or who incited "hatred or dislike of the Person of His Majestie or the established Government" was to be disqualified from holding any public, military or ecclesiastical office. This measure was expressed only to last during the King's lifetime. A permanent offence was committed by any person who claimed that the Long Parliament had not been dissolved, or that anyone had a duty to obey an oath to change the government, or that Parliament could legislate without the king's assent.
Sections 1 and 2 were repealed on 28 January 1863 by section 1 of, and the Schedule to, the Statute Law Revision Act 1863. There were some other repeals over the years, and then the whole Act, so far as unrepealed, was repealed by the Criminal Law Act 1967.
