But if it pleases you to hear what other saints also might think about this birthday, hear David speaking, "In iniquity I was conceived and in sins my mother brought me forth," showing that every soul which is born in flesh is polluted by the filth "of iniquity and sin"; and for this reason we can say what we already have recalled above, "No one is pure from uncleanness even if his life is only one day long." To these things can be added the reason why it is required, since the baptism of the Church is given for the forgiveness of sins, that, according to the observance of the Church, that baptism also be given to infants; since, certainly, if there were nothing in infants that ought to pertain to forgiveness and indulgence, then the grace of baptism would appear superfluous.
Quod si placet audire quid etiam alii sanctii de ista nativitate senserint, audi David dicentem: "In inquitatibus, inquit, conceptus sum, et in peccatis peperit me mater mea;" ostendens quod quaecunque anima in carne nascitur, iniquitatis et peccati sorde polluttur; et propterea dictum esse illud, quod jam superius memoravimus: quia "nemo mundus a sorde, nec si unius diei sit vita ejus." Addi his etiam illud potest, ut requiratur quid causae sit, cum baptisma Ecclesiae pro remissione peccatorum detur, secondum Ecclesiae observantiam etiam parvulis baptismum dari; cum utique si nihil esset in parvulis quod ad remissionem deberet et indulgentiam pertinere, gratia baptismi superflua videretur.
Origen (as translated/edited to Latin by Rufinus, English translation by Gary Wayne Barkley), Homily 8 on Leviticus (written c. A.D. 238-44, trans. c. A.D. 403-05), at section 3, paragraph (5) (pp. 157-8) (Latin in Migne PG, vol. 12, col. 496, A-B).