Yep, it's the last section of the Epistle to Diognetus. Check post on §11 for more info.

As mentioned when discussing §11, §§11-12 appear to be appended onto the Epistle to Diognetus, with the original ending lost.

This section is decidedly different, focusing on "knowledge" and hearkening back to motifs from Genesis regarding the tree of knowledge and the tree of life. Allegorical lessons are drawn from the imagery from Genesis; we actually get a glimpse into how text and concept were applied -- though since we don't have the whole picture (and since I'm not giving this much time at all) conclusions cannot be made here.

Since I'm out of my depth and not too motivated, I'll simply quote Meecham on this "knowledge":

In §1-10 the concept of knowledge as such finds little place. It is sufficient for the author to insist that the knowledge of God cannot be reached by man. It is given by God Himself through faith and begets fullness of joy (EpDiog 10.3). In §11-12, however, knowledge looms large. Faith is intimately conjoined with gnosis. In the fertile world of Christian life both the tree of knowledge and the tree of life are found. 'The tree of knowledge does not kill; but disobedience kills' (EpDiog 12.2). That is, gnosis has its due place in the religious life. On the other hand, its place is subordinate. God planted first in the Garden the tree of life, the path to which was indicated by the tree of knowledge. 'For there can be neither life without knowledge nor sound knowledge without true life. Wherefore each (tree) stands planted near the other' (EpDiog 12.4). Hence the Apostle (Paul) blamed the gnosis which is divorced from the truth that lead to life (1Co 8.1). Dorner finds "all through the twelfth chapter the pursuit of a middle path between Gnosticism and abstract piety". The author of this Appendix (§11-12) sets high value upon a true gnosis as an essential element inthe Christian life. Christianity is the highest philosophy and is in accord with reason. Yet it is revealed to faith, apprehended only by men enlightened by God, and attested by life. (Meecham 52-53)

And with that, I'll end my series of posts on the Epistle to Diognetus. Next up: Didache, though probably not until the new year. That is, unless there is a groundswell in voting and another document rises to the top of the poll.