Ante-Nicene Fathers, Vol VII:
Lactantius: Chap. XX.—Of himself and the truth

Chap. XX.—Of Himself and the Truth.

These things I have written to you, Demetrianus, for the present in few words, and perhaps with more obscurity than was befitting, in accordance with the necessity of circumstances and the time, with which you ought to be content, since you are about to receive more and better things if God shall favour us. Then, accordingly, I will exhort you with greater clearness and truth to the learning of true philosophy. For I have determined to commit to writing as many things as I shall be able, which have reference to the condition of a happy life; and that indeed against the philosophers, since they are pernicious and weighty for the disturbing of the truth. For the force of their eloquence is incredible, and their subtlety in argument and disputation may easily deceive any one; and these we will refute partly by our own weapons, but partly by weapons borrowed from their mutual wrangling, so that it may be evident that they rather introduced error than removed it.  

Perhaps you may wonder that I venture to undertake so great a deed. Shall we then suffer the truth to be extinguished or crushed? I, in truth, would more willingly fail even under this burthen. For if Marcus Tullius, the unparalleled example of eloquence itself, was often vanquished by men void of learning and eloquence,—who, however, were striving for that which was true,—why should we despair that the truth itself will by its own peculiar force and clearness avail against deceitful and captious eloquence? They indeed are wont to profess themselves advocates of the truth; but who can defend that which he has not learned, or make clear to others that which he himself does not know? I seem to promise a great thing; but there is need of the favour of Heaven, that ability and time may be given us for following our purpose. But if life is to be wished for by a wise man, assuredly I should wish to live for no other reason than that I may effect something which may be worthy of life, and which may be useful to my readers, if p. 300 not for eloquence, because there is in me but a slight stream of eloquence, at any rate for living, which is especially needful. And when I have accomplished this, I shall think that I have lived enough, and that I have discharged the duty of a man, if my labour shall have freed some men from errors, and have directed them to the path which leads to heaven.