Although the debate of the authenticity of the letters attributed to the desert fathers is one of the most important issues related to the recent research on early monasticism, an important part of these texts, the so-called ‘Macarian letters' have only occasionally caught some scholarly attention. This paper examines the most important piece of the corpus, the first letter usually cited as ‘Ad filios Dei'. In the first part of my paper I present a philologico-critical analysis of the different recensions of the letter with particular emphasis on the Greek, Coptic and Syriac versions, and then I turn to survey the central theological and ascetical motifs of the text (the role of the Holy Spirit in ascetic life, self-knowledge, and the ‘play of the divine grace') and examine their possible relationship with monastic literature in the context of the letters of Antony and Ammonas. With the help of this duplex investigation I try to make a decision about the question of authenticity in stricter and in a more general sense as well: ‘Is it possible that Macarius the Great himself was the author of the letter or not?' or ‘whether the Ad filios Dei be a credible witness of 4th century monasticism, or it is just a later pseudonym compilation?'