In natali Domini is the fifth of six sermons Ælfric wrote for Christmas over the course of his career, and its treatment of the natures of the triune Creator and the human soul constitutes the fullest tandem treatment of these doctrines in his corpus. He had taken up these topics in Christmas sermons written at roughly five-year intervals beginning around 990 with his Latin Sermo in natale Domini et de ratione anime (AH I.1), then in Old English in the Lives of Saints (LS I.1), and again in the vernacular in In natali Domini. The celebration of Christ's incarnation provided Ælfric with an opportunity to discuss the nature of not only the Son but the Father and Holy Spirit as well, and their coeternality and consubstantiality lead him to consider the eternality of the soul. As he says in In natali Domini, the Incarnation set in motion Christ's redemption of a great company of souls to dwell with him in heaven [lines 1–22], so it is essential that humans think on heavenly things. For Ælfric, a proper understanding of the Trinity [lines 23–109], the orders of created beings [lines 110–90], and the soul [lines 191–424] contributes to the proper exercise of one's faculties. To that end, he teaches about the soul's creation in God's image [lines 191–230], its capacities (desire, anger, reason [lines 231–60]), Trinitarian likeness (it has memory, understanding, will [lines 261–313]), virtues (wisdom, righteousness, moderation, steadfastness [lines 314–38]), and work (as soul, spirit, understanding, mind [lines 347–75]). It is, above all, invested with dignity [lines 376–400], and heavenly wisdom its highest desire [lines 401–20]. Ultimately, Ælfric argues that the rational soul exhibits true wisdom when it desires salvation, the true life in heaven people earn by loving, honoring, and learning the things that are of God and pleasing to him. The sermon's final soteriological turn serves as a reminder that for all its theological abstractness, the work is chiefly pastoral. On the day Christ came into the world, Ælfric asks the audience to consider their inevitable departure from it.

Rather than a new composition, In natali Domini is a rewriting of the Christmas sermon Ælfric had composed for the Lives of Saints (LS I.1) between 993 and 998.