This is an anonymous letter written about 150 A.D. The author is describing early Christians to a distinguished and noble pagan gentleman (in some places it is said that the author is not anonymous but Mathetes).

The author of the letter writes,

“The Christians are distinguished from others neither by country, nor language, nor the customs which they observe, for they neither inhabit cities of their own, nor employ a particular form of speech. They follow the customs of the citizens of whatever country they inhabit in regard to clothes, or food, or the rest of their ordinary conduct."

Indeed I received the text of this Letter to Diognetus from my friend in an attempt to explain me what makes a believer different from a person that practices an ethics that is externally indistinguishable from that of the believer.

However, Diognetus's letter, considered just beautiful in some aesthetical scales, is pretty clear in this particular point: "you must not hope to learn the mystery of their peculiar mode of worshipping God from any mortal".

In a word, do not expect that they will be able to explain themselves.