Justin Martyr, answered from the texts and cited to the page.
Christ is the first-born of God, and I have declared that He is the Word of whom every race of men were partakers. Those who lived reasonably are Christians, even though they were thought atheists: among the Greeks, Socrates and Heraclitus and men like them, and among the barbarians, Abraham and Ananias and Azarias and Mishael and Elias.1 Yes, I call them that, and I mean it of the reason they lived by, not of any rite they performed.
But observe what they had, and what they lacked. Whatever lawgivers or philosophers uttered well, they worked out by finding and contemplating some part of the Word. Because they did not know the whole of the Word, which is Christ, they often contradicted themselves.2 The Stoics were admirable in their moral teaching, as the poets were in some particulars, on account of the seed of reason implanted in every race of men.3
A seed is not the fruit. Christ, who appeared for our sakes, became the whole rational being, body and reason and soul, and that is why our doctrines are greater than all human teaching.2 Take Socrates, who was most zealous in this direction. He was accused of the very crimes laid against us, of bringing in new divinities and refusing the gods the state recognised.
He cast Homer and the poets out of the city and taught men to reject the wicked demons, and he urged them to seek the unknown God by the investigation of reason. He said it is neither easy to find the Father and Maker of all, nor safe to declare Him to everyone once found.4 Yet no one trusted in Socrates so as to die for his doctrine. In Christ, who was partially known even by Socrates, for He was and is the Word who is in every man, not only philosophers and scholars believed, but artisans and the entirely uneducated, despising glory and fear and death.
He is a power of the ineffable Father, not the mere instrument of human reason.4 That is the difference between the part and the whole. The demons make the same point in their own way. They have always contrived that all who live a reasonable and earnest life and shun vice be hated, and Heraclitus and Musonius were hated and put to death. It is no wonder that they hate more bitterly those who live not by a part of the diffused word but by the knowledge and contemplation of the whole Word, which is Christ.5
As to the second half of your question: when I had gone through the schools and the old man had spoken to me, a flame was kindled in my soul, and a love of the prophets and of those men who are friends of Christ possessed me. Revolving his words, I found this philosophy alone to be safe and profitable, and for that reason I am a philosopher.6
Trypho, when I told him so, urged me to abide in the philosophy of Plato or some other man, cultivating endurance, self-control and moderation, and called Christ a groundless report.7 I do not despise endurance and self-control, and I grant that Plato and the Stoics taught them well. But they are safe only as parts, and safe altogether only in the Word who gathers them.
The Word of God, who is Christ, was sown in every race of men, and whoever lived by reason lived by him: Socrates and Heraclitus among the Greeks, Abraham and the three youths among the barbarians. Each philosopher spoke well in proportion to his share of the seed, and what he said well belongs to the Christians; where the schools contradict one another they lacked the whole.
Plato also borrowed from Moses. The gospel is not a departure from philosophy but the one philosophy that is safe and profitable, because the whole Word became man.89101112
We have been taught that Christ is the first-born of God, and we have declared above that He is the Word of whom every race of men were partakers; and those who lived reasonably are Christians, even though they have been thought atheists; as, among the Greeks, Socrates and Heraclitus, and men like them; and among the barbarians, Abraham, and Ananias, and Azarias, and Mishael, and EliasThe First Apology
For whatever either lawgivers or philosophers uttered well, they elaborated by finding and contemplating some part of the Word. But since they did not know the whole of the Word, which is Christ, they often contradicted themselves.The Second Apology
on account of the seed of reason [the Logos] implanted in every race of menThe Second Apology
no one trusted in Socrates so as to die for this doctrine, but in Christ, who was partially known even by Socrates (for He was and is the Word who is in every man...The Second Apology
the devils have always effected, that all those who anyhow live a reasonable and earnest life, and shun vice, be hated.The Second Apology
a flame was kindled in my soul; and a love of the prophets, and of those men who are friends of Christ, possessed me; and whilst revolving his words in my mind, I found this philosophy alone to be safe and profitable. Thus, and for this reason, I am a philosopher.Dialogue with Trypho
it were better for you still to abide in the philosophy of Plato, or of some other man, cultivating endurance, self-control, and moderation, rather than be deceived by false wordsDialogue with Trypho
But we are not concerned, since we know that God is a just observer of all. But would that even now some one would mount a lofty rostrum, and shout with a loud voice; "Be ashamed, be ashamed, ye who charge the guiltless with those deeds which yourselves openly commit, and ascribe things which apply to yourselves and to your gods to those who have not even the slightest sympathy with them.The Second Apology
We know that the wicked angels appointed laws conformable to their own wickedness, in which the men who are like them delight; and the right Reason, when He came, proved that not all opinions nor all doctrines are good, but that some are evil, while others are good. Wherefore, I will declare the same and similar things to such men as these, and, if need be, they shall be spoken of more at large.The Second Apology
And if you also read these words in a hostile spirit, ye can do no more, as I said before, than kill us; which indeed does no harm to us, but to you and all who unjustly hate us, and do not repent, brings eternal punishment by fire. Chapter XLVI.--The Word in the world before Christ.The First Apology
But pray that, above all things, the gates of light may be opened to you; for these things cannot be perceived or understood by all, but only by the man to whom God and His Christ have imparted wisdom.' Chapter VIII.--Justin by his colloquy is kindled with love to Christ.Dialogue with Trypho
And God said, Let there be light; and it was so." So that both Plato and they who agree with him, and we ourselves, have learned, and you also can be convinced, that by the word of God the whole world was made out of the substance spoken of before by Moses. And that which the poets call Erebus, we know was spoken of formerly by Moses. Chapter LX.--Plato's doctrine of the cross.The First Apology
His signature doctrine and the opposite of Tertullian's answer; read as the charter of Christian humanism, and questioned whether it is Platonism wearing a Christian name or a Christian criterion passing judgement on Plato.