In his so-called Second Apology, Justin focuses his attention on the relationship between Christianity and philosophy. Here Justin argues that Christianity is better than Platonism or Stoicism because Christ revealed directly and in ordinary ways what the philosophers had only glimpsed and expressed in a convoluted manner (10).
One thing we see in the Second Apology is that the Apologists often challenged the current pantheon of deities by reminding their listeners that the great philosophers, whom they esteemed so highly, rejected such polytheism, just as did the Christians. Justin uses Socrates as his example (10).
Justin explains the similarities between Christianity and Greek philosophy through his understanding of the Logos, the Greek term for "Word." The Greeks understood the term as the principle of Reason. Justo Gonzalez states, "According to a tradition of long standing in Greek philosophy, the human mind can understand reality because it shares in the Logos or the universal reason that undergirds all reality. [I]f we are able to understand that two and two make four, the reason for this is that both in our minds and in the universe there is a Logos, a reason or order according to which two and two always make four" (The Story of Christianity, 1.55).
So the doctrine of the Logos is not Justin's creation. He seems to borrow much of his understanding from Philo of Alexandria. The concept of the Logos is found in the Prologue of John's Gospel, though with a decidedly more Jewish tone. The Logos in John is employed in reference to the divine and pre-existent nature of the Savior. Justin concludes that all knowledge is a gift from Christ and that those who lived "reasonably" were Christians (First Apology, 46).
The philosophers, says Justin, knew the Word, but only partially. They knew the truths that the Word had revealed to them, but they did not know the Word itself. The Word was "made flesh" and Christians know the Word entirely in Christ. Christians behold as a whole what the ancient philosophers could only see in part (Second Apology, 8). Philosophy contains only part of the truth. It cannot discover the whole truth on its own, but only if given by the Logos himself. Often, philosophy cannot distinguish between the part of the truth it has and the falsehood that surrounds it. Christians, however, can distinguish between the true and false elements in philosophy because they know the truth incarnate (First Apology, 54-55, 59).
The Apologists #1: Introduction
The Apologists #2: Pagan Culture and Judaism
The Apologists #3: Justin Martyr, c. 100-165, part 1