Lactantius — Divine Institutes 4.25
Let people learn and understand why the highest God, when he was sending his legate and messenger to instruct mortality with the precepts of justice, willed him to be clothed with flesh and afflicted with torment and punished by death. For since there was no justice on earth, he sent a teacher as a living law, to found a new name and temple, to sow true and pious worship through the whole earth by words and by example.
Yet so that it would be certain he was sent by God, he did not need to be born as a human is born, compounded from both mortal parents; but so that it would appear that even in the human he was heavenly, he was created without a father’s agency. For he had God as the spiritual father — and as the father of his spirit is God without a mother, so the mother of his body is a virgin without a father.
He was therefore both God and human, set as middle between God and human — which is why the Greeks call him mediator — so that he might be able to lead the human to God, that is, to immortality. Because if he had been God only, as said above, he could not offer humans examples of virtue; if human only, he could not compel humans toward justice unless authority and power greater than a human were added.
Latin
Discant igitur homines et intellegant quare deus summus cum legatum ac nuntium suum mitteret ad erudiendam praeceptis iustitiae mortalitatem, uoluerit eum carne indui et cruciatu adfici et morte multari. nam cum iustitia nulla esset in terra, doctorem misit quasi uiuam legem, ut nomen ac templum nouum conderet, ut uerum ac pium cultum per omnem terram et uerbis et exemplo seminaret. sed tamen ut certum esset a deo missum, non ita illum nasci oportuit, sicut homo nascitur ex mortali utroque concretus, sed ut appareret etiam in homine illum esse caelestem, creatus est sine opera genitoris. habebat enim spiritalem patrem deum et sicut pater spiritus eius deus sine matre, ita mater corporis eius uirgo sine patre. fuit igitur et deus et homo, inter deum atque hominem medius constitutus, unde illum Graeci μεσίτην uocant, ut hominem perducere ad deum posset id est ad inmortalitatem: quia si deus tantum fuisset, ut supra dictum est, exempla uirtutis homini praebere non posset, si homo tantum, non posset homines ad iustitiam cogere, nisi auctoritas ac uirtus homine maior accederet.