In the third theorem of his Theologia naturalis Alsted discusses the issue of non-Christian authors and commends their use even for public preaching, though with certain caveats:
III. Præclara ethnicorum dicta Theologica ab eis tanquam iniustis possessoribus, in usum nostrum transferenda sunt.
Hoc theorema totidem ferme verbis occurrit apud Augustinum lib. 3. de doctrina Christ. cap, 4: cum quo magnam cognationem habet illud, quod adscribitur Maximiliano I. Ethinici audiendi sunt, non tanquam philomelæ, sed tanquam ranæ. His dictis monemur, non nimium tribuendum esse ethnicis scriptoribus, sed ita illos legendos, ut ne subsistamus in illorum scriptis, sed vera a falsis discernamus, & vera ab iis tanquam iniustis possessoribus ad usum nostrum transferamus, id quod fiet, si pæclaras ipsorum sensentias accumulemus, ut iis pudorem incutiamus multis Christianis segniter & frigide de rebus divinis cogitantibus & loquentibus. Hunc usum velim diligenter notare concionatores, qui auditoribus suis obstinatis subinde huiusmodi formulas proponere debent: Audite Ciceronem, quem natura hoc docuit. Item: Exurget in illo die ethnicus ille, ac vos damnabit. Item: Pudor est nos, qui Christianorum nomen præ nobis ferimus, superari ab ethnicis.
III. The admirable theological maxims of the heathen should be removed from them (as from wrongful possessors) for our use.
This theorem is stated by Augustine in nearly the same words in book 3, chapter 4 of De Doctrina Christiana. 1 Together with this great concept [of Augustine’s] is that which is written by Maximilian I, “They should listed to the heathen, not as to nightingales, but as to frogs.” 2 With these words we are advised that not too much should be attributed to the writings of the heathen but they are to be read in such a way that we do not support ourselves by their writings but that we discern the true from the false. Also, we should remove what is true from their [writings] (as from wrongful possessors) for our use, and it will come to pass, if we shall store up their admirable conclusions, that by means of them we may instill shame in many Christians [who are] sluggish and cold in their thoughts and speech concerning divine things. I want preachers to diligently make note of this use, who ought to repeatedly propose rules such as the following to their obstinate listeners: “Listen to Cicero, to whom nature taught this.” Likewise, “The heathen will rise up in the day [of judgment] and condemn you.” Finally, “We should be ashamed, who carry the name of ‘Christian’ before us, to be surpassed by the heathen.”