154. The principle of legality constitutes one of the central elements of criminal prosecution in a democratic society by establishing that “no one shall be convicted for any act or omission that did not constitute a criminal offense, under the applicable law, at the time it was committed.” This principle guides the actions of all the organs of the State, in their respective jurisdictions, particularly in the case of the exercise of punitive powers. 272 This Court has also indicated that the principle of non-retroactivity means that a person may not be punished for an act that, when it was committed, was not an offense or could not be prosecuted or punished. 273 155. If, after an offense has been committed, the law provides for the imposition of a lighter punishment, the guilty person must benefit from this. The principle of the retroactivity of the criminal law is established in the last part of Article 9 of the Convention, which indicates that “[i]f, subsequent to the commission of the offense the law provides for the imposition of a lighter punishment, the guilty person shall benefit therefrom.” 274 This norm should be interpreted in good faith, in accordance with the ordinary meaning to be given to the terms of the treaty in their context and in the light of the object and purpose of the American Convention, which is the effective protection of the individual, 275 as well as by an evolutive interpretation of the international instruments for the protection of human rights. 276 156. In this case, Messrs. Mémoli were convicted of the offense of defamation defined in article 110 of the Criminal Code in force at the time. In 2009, this norm was amended so that it excluded statements related to matters of public interest of that were not affirmative from any criminal punishment, and also eliminated the punishment of imprisonment for their perpetration (supra para. 91). In this way, the new definition of the offense of defamation removed the criminal nature of statements relating to matters of public interest or statements that were not affirmative, and reduced the criminal sanctions for the offense of defamation. 157. This Court notes that there were two main reasons why the review of the criminal conviction imposed on Messrs. Mémoli was rejected: (i) because the punishment had concluded, in the opinion of the Appellate Chamber and (ii) because no evidence had been provided concerning “the facts that resulted in the conviction, in order to corroborate that they involved a supposed impairment of the right to freedom of expression on issues of significant public interest,” or concerning the “existence of a claim for compensation against the appellants,” in the opinion of one of the judges of the Supreme Court of Justice of Buenos Aires (supra paras. 93 and 94). 158. The Court recalls that, in this case, the domestic judicial authorities concluded that the information contained in the statements made by Messrs. Mémoli was not of public interest (supra paras. 145 to 147). Based on this, the decriminalization of statements relating to matters of public interest would not apply to the sentence imposed on Messrs. Mémoli, because the statements were not “related to a matter of public interest,” as established in the 272 Cf. Case of Baena Ricardo et al. v. Panama. Merits, reparations and costs. Judgment of February 2, 2001. Series C No. 72, para. 107, and Case of Mohamed v. Argentina. Preliminary objection, merits, reparations and costs. Judgment of November 23, 2012. Series C No. 255, para. 130. 273 131. 274 Cf. Case of Ricardo Canese v. Paraguay, supra, para. 175, and Case of Mohamed v. Argentina, supra, para. Cf. Case of Ricardo Canese v. Paraguay, supra, para. 178. 275 Cf. Case of the 19 Tradesmen v. Colombia. Merits, reparations and costs. Judgment of July 5, 2004. Series C No. 109, para. 137, and Case of Artavia Murillo et al. (In vitro fertilization) v. Costa Rica, supra, para. 173. 276 Cf. Case of Ricardo Canese v. Paraguay, supra, para. 178, and Case of Artavia Murillo et al. (In vitro fertilization) v. Costa Rica, supra, para. 173. 60