III. Justice required that Satan should be vanquished by God made man
And, dearly beloved, this very fact that Christ chose to be born of a Virgin does it not appear to be part of the deepest design? I mean, that the devil should not be aware that Salvation had been born for the human race, and through the obscurity of that spiritual conception, when he saw Him no different to others, should believe Him born in no different way to others. For when he observed that His nature was like that of all others, he thought that He had the same origin as all had: and did not understand that He was free from the bonds of transgression because he did not find Him a stranger to the weakness of mortality.
For though the true mercy of God had infinitely many schemes to hand for the restoration of mankind, it chose that particular design which put in force for destroying the devil's work, not the efficacy of might but the dictates of justice. For the pride of the ancient foe not undeservedly made good its despotic rights over all men, and with no unwarrantable supremacy tyrannized over those who had been of their own accord lured away from God's commands to be the slaves of his will.
And so there would be no justice in his losing the immemorial slavery of the human race, were he not conquered by that which he had subjugated. And to this end, without male seed Christ was conceived of a Virgin, who was fecundated not by human intercourse but by the Holy Spirit. And whereas in all mothers conception does not take place without stain of sin, this one received purification from the Source of her conception. For no taint of sin penetrated, where no intercourse occurred. Her unsullied virginity knew no lust when it ministered the substance. The Lord took from His mother our nature, not our fault. The slave's form is created without the slave's estate, because the New Man is so commingled with the old, as both to assume the reality of our race and to remove its ancient flaw.
Source: Sermons (New Advent)