These books should contain: —an explanation of the message of salvation (constant reference must be made to the sources, and a clear distinction must be kept between those things which pertain to the faith and to the doctrine that must be held, and those things which are mere opinions of theologians); —psychological and pedagogical advice; —suggestions about methods. Books and other printed materials intended for study and activity by those being taught should also be provided. These printed materials can be made part of the books for the use of those being taught, or they can be published as separate booklets. Finally, cane should be taken to publish books for the use of parents, if the question is one of giving catechesis to children.