This paper studies a probable reference to Justin (an ancient Greek apologist) in Montesquieu’s Lettres Persanes. In letter 33, the Persian Usbek discusses the possibility of salvation for Christians: in our opinion, this passage (and especially the expression “semence de nos dogmes”) turns against Christians Justin’s conception of Λόγος σπερματικός. After a short presentation of ancient Christian apologetics and of Justin’s thought, we will point out that Montesquieu had a good knowledge of patristic literature and a copy of Justin’s works in his library. Then we will see that Justin played a key-role in the seventeenth-century debate on the possibility for pagans to be saved (“querelle de la vertu des païens”) – a problem that Montesquieu discussed in a lost work, and that is the real object of letter 33. Finally, we will study the letter 33 itself. We will show that the allusion to Justin strengthens the irony and the ambiguity of the text: precisely because of Usbek’s use of Christian expressions, this letter challenges not only Christian, but also Muslim religious primacy.