Why are some books in the Bible while others were excluded and how was that decided?
By the end of the first century or early in the second century, all of the texts that now make up the New Testament had been written. As early as AD 95 or 96, Clement of Rome alludes to multiple passages in the New Testament, though he does not name them as scripture yet (that is a title still reserved for the Old Testament at this point). In the mid-second century, St. Justin Martyr refers to the gospels as the “memoirs of the apostles” in his First Apology , and in the famous text Against Heresies of St. Irenaeus (c. 180), the saint firmly proclaims the authority of the four gospels. At the same time, a number of other texts were circulated amongst the nascent Christian community. There are really two categories of texts outside of the canon of Scripture . First, texts whose teachings were accepted in whole or in part as edifying for the Christian community but ultimately deemed outside of the core texts which became the New Testament, and second, texts that were cons...