The Catechism of the Catholic Church cites the decree Dei Verbum of Vatican II which states this fundamental principle as the proper manner to view the articles of faith. The Catechism underlines the necessity of following this norm in order to assure that Scripture be appropriately interpreted. Different as the books which comprise it may be, Scripture is a unity by reason of the unity of God's plan, of which Christ Jesus is the center and heart, open since his Passover. This notion that the heart of the Scriptures and its center is Christ Jesus is a profound insight that makes the interpretation of the inspired word a living encounter with the Savior. St. Thomas Aquinas is the one who had originally stated the principle of the unity of Scripture in these terms as the Catechism acknowledges in citing him.
Heresy is a term taken from the Greek word meaning to choose or select.
It means that the revealed truth is diminished in its fullness though an arbitrary and mistaken selection from the totality. The word itself suggests incompleteness. The error envisaged by the term heresy is that of incompleteness. When any one refuses consent to some essential point of Catholic belief that person is making an heretical choice and ceases to believe according to Catholic faith. Of course, that does not yet make such a one a heretic formally in the canonical sense.