421. The effects of the Code on the older legislation are as follows:
(a) it retains in their entirety liturgical laws that are not expressly corrected; agreements of the Holy See with various nations, even if they are opposed to the Code; favors, privileges and indults that are not revoked (Canons 2-4);
(b) disciplinary laws of ecclesiastical origin opposed to the Code are to be held as revoked, even if they are particular, unless the contrary is provided. Disciplinary laws of ecclesiastical origin omitted by the Code are retained in force, if they are particular; they are abrogated, if they are general and not contained at least implicitly in the Code; if a general law decreed a penalty, it must be expressly mentioned in the Code to retain force (Canon 6);
(c) customs, universal or particular, opposed to the Code, when expressly disapproved by it, must be corrected, even if immemorial; when they are not expressly disapproved by the Code, they may or may not be continued, as a rule, according as they are immemorial—or one century old—or not (Canon 5).
422. The rules laid down for the interpretation of the Code are as follows: (a) in those parts where the Code agrees with the older legislation, it is to be interpreted by means of the latter; (b) in those parts where it certainly disagrees with the older legislation, it is to be interpreted from its own phraseology (Canon 6).
423. Lawgivers in the Church.—The Pope, as Vicar of Christ and Visible Head of the Church, has supreme legislative power in the Church (Canon 218): “Thou art Peter, and upon this rock I will build My Church .... And I will give to thee the keys of the kingdom of heaven, etc.” (Matt., xvi. 18, 19). Thus, the Pope can legislate: (a) for the whole Church, either alone or with the body of the Episcopate subject to him in an Ecumenical Council, either directly or through Congregations; (b) for any part of the Church, either directly or through representatives. Thus also, by Papal concession, legates may legislate for a place to which they are sent, _Praelati nullius_ for a territory over which they are placed, General Chapters for a Religious Order, and the like.
424. The Bishops, “placed by the Holy Ghost to rule the Church of God” (Acts, xx. 28), have legislative power within their own territory, dependently on the Pope (Canon 335). (a) They can make laws, each for his own diocese, either in or out of a synod; (b) when gathered together in council, provincial or plenary, they can legislate for ecclesiastical provinces, or for all the faithful of their country.
425. Subject-Matter of Church Law.—The end of the Church being the glory of God and the salvation of souls, she can legislate concerning all matters that are sacred or that refer, directly or indirectly, to the satisfaction of man or the worship of God (see Leo XIII, Const. _Immortale Dei_, d. 1 Nov. 1885).
(a) The Church can call to mind those things that are already prescribed by the Divine Law, Natural or Positive; and, although she cannot dispense in these laws (see 313-814 and exception as to hypothetical positive law in 357), she can interpret them authoritatively, and can decide when obligations of the Divine Law, that depend upon an act of the human will, cease (see 315-316).
(b) The Church can determine those things that were left undetermined in the Divine Law. Examples: The manner in which the Lord’s Day is to be sanctified, the times and frequency with which the Divine law of Communion is to be fulfilled, the way in which the obligation of fasting is to be complied with, etc.