The Enchiridion by St. Augustine
Chapter 44.--In Speaking of Sin, the Singular Number is Often Put for the Plural, and the Plural for the Singular.
But even these latter are frequently said to die to sin, though undoubtedly they die not to one sin, but to all the numerous actual sins they have committed in thought, word, or deed: for the singular number is often put for the plural, as when the poet says, |They fill its belly with the armed soldier,| though in the case here referred to there were many soldiers concerned. And we read in our own Scriptures: |Pray to the Lord, that He take away the serpent from us.| He does not say serpents though the people were suffering from many; and so in other cases. When, on the other hand, the original sin is expressed in the plural number, as when we say that infants are baptized for the remission of sins, instead of saying for the remission of sin, this is the converse figure of speech, by which the plural number is put in place of the singular; as in the Gospel it is said of the death of Herod, |for they are dead which sought the young child's life,| instead of saying, |he is dead.| And in Exodus: |They have made them,| Moses says, |gods of gold,| though they had made only one calf, of which they said: |These be thy gods, O Israel, which brought thee up out of the land of Egypt,| -- here, too, putting the plural in place of the singular.