Inspiration comes from the Latin inspirare, meaning “to breathe upon.” The Greek word for inspiration is theopneustos, meaning literally “God-breathed”; in Hebrew, one can use either neshamah or ruwach. Divine Inspiration is understood to mean that God directly influenced each and every sacred author to write without interfering with his individual free will. Pope Leo XIII gave the best definition when he said, “Inspiration consists in that supernatural influence by which God so arouses and directs the sacred authors to write, (and) assists them in writing, so that all and only what He Himself wills do they correctly formulate in their minds, determine to write faithfully, and express aptly in an infallibly truthful manner” (Providentissimus Deus, #20, 1893).
What inspiration is not is verbal dictation (the Holy Spirit did not give an interview); nor is it subsequent approbation, where the author submits written material for divine approval (God is not an editor, either); nor is it only an ideological inspiration on just moral or spiritual matters. Unlike infallibility, which is a negative charism (spiritual gift) that prevents the source from teaching error, inspiration is a positive charism whereby the sacred author writes only what God wants and how He wants it, while still retaining his free will.
Inspiration, since it comes directly from God, is also infallible (free of moral or doctrinal error) and inerrant (free of mistakes) to protect the integrity and sanctity of the revealed message. The human authors, however, are free to convey the inspired word using vocabulary and idioms known by the person.
Only inspired text is allowed to be in the Bible and only divinely inspired text can contain divine revelation. Books which did not get into the Bible, like the Gospel of Mary Magdalene or the Apocalypse of Moses, are not considered inspired. One mistake some people make is to conclude that only inspired text is true. Being that they originate from God, all inspired books are true and inerrant. Yet, other human texts can be and are true but are not considered inspired. A phone book is true in that the numbers and names are accurate. A chemistry book is true in that water is two parts hydrogen and one part oxygen. However, neither of these is inspired text.
It is not the author, the subject, nor the date of the document which makes it inspired. If Saint Joseph, the husband of the Virgin Mary and foster father of Jesus, had written something and archeologists found it tomorrow, it would not be considered inspired just because of who wrote it, when it was written, or even what it contained. The commentaries of the Gallic Wars by the Roman Emperor Julius Caesar in which he says “Gallia est omnis divisa in partes tres” (All Gaul is divided into three parts) was written in 58 BC, which predates the New Testament, and is considered truthful but not inspired.
Only an authority can decide whether or not a book is inspired. The recognized authentic authority is the Church—especially the “teaching authority” of the Church, which is called the magisterium. The Bible itself never says which books belong in it or are considered inspired. The word Bible is not even in the Bible. Modern editors and publishers print a table of contents, but the original sacred authors never had such a list. The Church did not create the Bible but was the one to determine which books belonged in it and in what order, and is the one authorized to faithfully interpret the Bible as well.