Why did church fathers such as Irenaeus say the Shepard of Hermas is Scripture then?
Irenaeus says "Truly, then, the Scripture declared, which says, "First of all believe that there is one God, who has established all things, and completed them, and having caused that from what had no being, all things should come into existence. He who contains all things, and is Himself contained by no one." [Book 2, First Commandment, of the Shepherd of Hermas]. Rightly also has Malachi said among the prophets: "Is it not one God who hath established us? Have we not all one Father?" (4.20.2. of Adversus Haereses)