Athenagoras (d. ca. 185) was another early apologist whose work A Plea for the Christians is a beautifully written defense of the faith against accusations of atheism (among other things) leveled at Christians by their society. The text itself was likely written around AD 177 and shares similarities with those of Aristides and Justin Martyr; both in terms of its use of Greek philosophy and in addressing like charges. It was addressed to the emperors Marcus Aurelius and Commodus and appeals to their learning as philosophers.
What I’d like to highlight is a statement found in chapter 12, “Consequent Absurdity of the Charge of Atheism,” where Athenagoras gives us a very clear statement about the Trinity. This is a particularly useful quote against those who would argue that trinitarian doctrine is a later construct.
Are, then, those who consider life to be comprised in this, “Let us eat and drink, for tomorrow we die,” and who regard death as a deep sleep and forgetfulness (“sleep and death, twin brothers”), to be accounted pious; while men who reckon the present life of very small worth indeed, and who are conducted to the future life by this one thing alone, that they know God and His Logos, what is the oneness of the Son with the Father, what the communion of the Father with the Son, what is the Spirit, what is the unity of these three, the Spirit, the Son, the Father, and their distinction in unity; and who know that the life for which we look is far better than can be described in words, provided we arrive at it pure from all wrong-doing; who, moreover, carry our benevolence to such an extent, that we not only love our friends (“for if ye love them,” He says, “that love you, and lend to them that lend to you, what reward will ye have? “), shall we, I say, when such is our character, and when we live such a life as this, that we may escape condemnation at last, not be accounted pious?
Athenagoras, “A Plea for the Christians” in Ante-Nicene Christian Library, eds., Alexander Roberts and James Donaldson (Edinburgh: T and T Clark, 1867), 2:388.