Catechism of the Catholic Church §1854 — excerpt
Sins are rightly evaluated according to their gravity. the distinction between mortal and venial sin, already evident in Scripture, became part of the tradition of the Church.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Whole chapters cited: 1 John 16 · 1 John 17