The Highest Reason

Hoc ipsum autem, dilectissimi, quod Christus nasci elegit ex Virgine, nonne apparet altissimae fuisse rationis? Ut scilicet natam humano generi salutem diabolus ignoraret, et spiritali latente conceptu, quem non alium videret quam alios, non aliter crederet natum esse quam caeteros. Cujus enim similem cum universis advertit naturam, parem habere arbitratus est cum omnibus causam; nec intellexit a transgressionis vinculis liberum, quem ab infirmitate mortalitatis non invenit alienum. Verax namque misericordia Dei, cum ad reparandum humanum genus ineffabiliter ei multa suppeterent, hanc potissimum consulendi viam elegit, qua ad destruendum opus diaboli, non virtute uteretur potentiae, sed ratione iustitiae. Nam superbia hostis antiqui non immerito sibi in omnes homines ius tyrannicum vindicabat, nec indebito dominatu premebat, quos a mandato Dei spontaneos in obsequium suae voluntatis illexerat. Non itaque iuste amitteret originalem humani generis servitutem, nisi de eo quod subegent vinceretur. Quod ut fieret, sine virili semine conceptus est Christus ex virgine, quam non humanus coitus, sed Spiritus sanctus fecundavit. Et cum in omnibus matribus non fiat sine peccati sorde conceptio, haec inde purgationem traxit, unde concepit. Quo enim paterni seminis transfusio non pervenit, peccati se illic origo non miscuit. Inviolata virginitas concupiscentiarn nescivit, substantiam ministravit. Assumpta est de matre Domini natura non culpa. Creata est forma servi sine conditione servili, quia novus homo sic contemperatus est veteri, ut et veritatem susciperet generis, et vitium excluderet vetustatis. Sanctus Leo Magnus, Sermo XXII, Cap III, In Nativitate Domini II Source: Migne PL 54.196b-197a

And that, dearly beloved, Christ chose to be born of a Virgin, does it not appear to be have been a matter of the highest reason? That is, that the devil should be ignorant that Salvation had been born for the human race, and by the hidden nature of the spiritual conception, when he saw Him not unlike others, he should judge Him born in no different way to the rest. For when he observed that His nature was like that of all others, he deemed that He had the same origin as everyone else; he did not understand that He was free from the chains of transgression because he did not find Him a stranger to the weakness of mortality. For though the true mercy of God had unspeakably many ways for the restoration of mankind, it chose the way most potent for the destruction of the work of the devil, not by the use of the power of might but by the reason of justice. For the pride of the ancient foe not without reason had the right of tyrannous rule over all men, and with no unwarrantable supremacy lorded it over those who had been of their own accord lured away from God's commands to the service of his will. And so he would not justly lose the immemorial servitude of the human race, were he not conquered by that which he had subjugated. And that this be, without the seed of a man Christ was conceived of a virgin, she who was made fruitful not by human intercourse but by the Holy Spirit. And whereas in all mothers conception may not happen without the stain of sin, this one received purification from the source of her conception. For where there was no transfer of paternal seed, so there was no mixing with the origin of sin. Unviolated virginity knew no lust when it ministered to the substance. The Lord took from His mother the nature but not the fault. The form of the slave was fashioned without the servile condition, because the new man was so mixed with the old that it both assumed the reality of the race and excluded the ancient fault. Pope Leo the Great, from Sermon 22, Chap 3, On The Nativity Of The Lord