The apostolic fathers.
Primitive Christian texts just beyond the boundaries of canon.
There are several Christian texts whose primitive nature has earned their authors the collective name of apostolic fathers. Most of these texts are attributed. It ought to be emphasized that the category of apostolic fathers is artificial; there is no single ancient collection that contains all of these texts as a discrete set. This designation is merely a matter of modern convenience.
In addition to those I have listed above, several of what I would prefer to call apologetic texts are often counted among the apostolic fathers; I am referring to the epistle to Diognetus and the apologies of Quadratus and of Aristides.