The text known as the Letter to Diognetus was discovered in 1436 by a Catholic priest, Thomas d'Arezzo, in a fish market in Constantinople. The existence of the unique manuscript code, the absence of a reference to the author, and the lack of contemporary or later patristic references in the text led many researchers to consider the text as counterfeit, while others argued that it was written for purposes of impressing others.

In the introduction of the letter, the author poses three questions regarding which Diognetus desires to be better informed: ◆What is the identity of the God of Christians? ◆What is the source of this excessive love they show towards their neighbors? ◆Why did their faith appear so late in the world?

In answering these questions, the author will critique both idolatry and the erroneous perceptions of the Jews regarding the worship of God. Subsequently, he will refer to the way of life of Christians who, although they live on earth like everyone else, are already citizens of Heaven as they are members of the Church, and in the following chapters, he will present the Christian teaching about God while also critiquing the views of various philosophers.

Regarding the question of the delayed presence of the Christian faith in the world, the author will say that God's redemptive plan through His Son was realized when humanity realized that it could not be saved by itself.