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".

[5] 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 in 2004.

[8]Sir William Blackstone wrote in his Commentaries on the Laws of England: the principal reason, undoubtedly, is to secure the subject from being sacrificed to fictitious conspiracies, which have been the engines of profligate and crafty politicians in all ages.

The Act made it treason to: within the realm, or without, compass, imagine, invent, devise or intend death or destruction or any bodily harm tending to death or destruction, maim or wounding, imprisonment or restraint of the person of ... the Kingor, within the realm or without, compass, imagine, invent, devise or intend: These provisions were expressed only to have effect during the lifetime of the King, Charles II.

In some Commonwealth of Nations countries, such as Canada,[13] Australia[14] and New Zealand[15] it is still treason to imprison or harm the King.

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 (but could keep his peerage).

A peer who was convicted of any offence under the Act was to be disqualified from sitting in the House of Lords, unless pardoned (although he would keep his title).