Around about 90 BC Antiochus of Ascalon reacted against the skepticisms of the New Academy by founding what came to be called the Old Academy. The name 'Old Academy' and the peculiarity of its being newer than we call the New Academy is quite deliberate; Antiochus regarded it as intolerable that the School of Plato was so very un-Platonic, and thus advocated a return to Plato. Such reform movements are never merely reactionary; rediscovery in response to opposition to one's project is inevitably innovative. The new movement drew from Peripatetic and Stoic sources in order to defend itself, attack Academic skepticism, and form connections to the philosophical positions of other schools. Like any reform movement, it simultaneously identified itself with a past purity -- the original Academy of Plato and his first three successors -- and used this as a framework for developing new solutions to more contemporary problems. The resulting project was Platonic and Peripatetic and Stoic, which while eclectic is less eclectic than it is usually made to sound, because returning to the root made it possible to find genuine commonalities. The Peripatetics were an early offshoot of the Academy, through Aristotle; and many of the ideas we associate with Stoicism first found their basic form in the successors of Plato.
We don't have much on Antiochus, although what we do have suggests that he was better at articulating a vision than developing it. But the challenge was taken up by many others over the next several centuries, the most important of whom were Eudorus of Alexandria, Plutarch of Chaeronea (the best known of the three), and Philo of Alexandria. The third of these is especially intriguing and important, for Philo was a Jew (hence the other name he is known by, Philo Judaeus). Philo's integration of Jewish belief and Middle Platonism would set the stage for centuries to come. For Christianity partly grew up in the crucible of Middle Platonism, of the Philonic type, although in many cases it would not have been Philo himself so much as his general context and perhaps other Middle Platonists about whom we know less.
There are parts of Paul's letters and the Epistle to the Hebrews in which one might see such influence, but the influence is most marked in the Gospel of John (and it is notable that Hellenistic Jews come up explicitly a couple of times in the book, e.g., 7:35, 12:20ff). One of the most important means Philo had used to integrate Jewish and Greek thought was his developed account of the Logos. Logos, reason or word, was a common term in Greek philosophy, and was associated with the divine; it also occurs quite often in the Septuagint to refer to God's word. On Philo's account the Logos is a power in God; it is the divine Mind containing the Platonic Ideas and is active and effective as God's Word:
[I]n the one living and true God there were two supreme and primary powers, Goodness [or Creative Power] and Authority [or Regent Power]; and that by his Goodness he had created every thing; and that by his Authority he governed all that he had created; and that the third thing which was between the two, and had the effect of bringing them together was the Logos, for it was owing to the Logos that God was both a ruler and good.(Cher 1.27-28, qtd here)
It's the general Philonic idea of divine Logos that is in the background of the opening words of the Gospel of John:
In the beginning was the Logos
and the Logos was with God
and the Logos was God.
The same was in the beginning with God.
Through him all things were made;
without him nothing was made that has been made.
In him was life,
and that life was the light of men.
The light shines in the darkness,
but the darkness does not comprehend it.
And, he continues, the Logos became flesh and dwelt among us. Thus Christianity from its early days built on the vocabulary, or, rather, vocabularies, of Middle Platonism.
It is then unsurprising that many Middle Platonists found Christianity to have its attractions, and this brings us to the saint whose feast-day it is, Justin the Martyr. Justin was born to a pagan Greek family in Samaria (as he calls himself, he was "Justin, the son of Priscos, son of Baccheios, of Flavia Neapolis, in Palestinian Syria") in the early second century. He became a philosopher of Middle Platonist stamp, drawing now from Stoicism, now from Pythagoreanism, now from Aristotelianism, eventually making his way to Ephesus. He converted at some point to Christianity, in part, it seems, because he found them to be less morally objectionable than people in other schools of philosophy (particularly in terms of their courage and charity to others); and much of the argument in his works is that philosophy, especially as conceived by Socrates and Plato, finds its natural culmination in Christianity. Both philosophy and Christianity share in their love of Logos, but Christians hold that Logos so reciprocated as to become man. Justin explicitly mentions Socrates as a proto-Christian in his argument that it is unjust for the Empire to persecute Christians:
We have been taught that Christ is the first-born of God, and we have declared above that He is the Logos 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 Misael, and Elias, and many others whose actions and names we now decline to recount, because we know it would be tedious. So that even they who lived before Christ, and lived without Logos, were wicked and hostile to Christ, and slew those who lived according to Logos.
We don't know the full details of how it is that Justin became a martyr; certainly he was never hesitant to insist in public that Christians were right, although he is largely quite irenic when it came to pagans. There is some indication that in his wandering about first Ephesus and then Rome engaging in philosophical arguments he may have made enemies; he explicitly says at one point that he expected it to happen, and even named one Crescens, a Cynic philosopher, as a likely person to cause problems for him. Thus Crescens perhaps did get to him, or someone like Crescens, although we don't really know for sure. We do know that he was tried with six others in a standard trial under the prefect Rusticus during the persecutions under Marcus Aurelius and was put to death. It is perhaps an irony that the parataxis, the obstinacy, that the philosopher Marcus Aurelius loathed in Christians, the philosopher Justin exalted as showing the paradoxical summit of a good life, a life that put truth before life. The emperor probably never knew the teacher, but for different reasons they would both have considered Justin's death appropriate to his views.
Justin lived in the evening of Middle Platonism. In the very next century people like Plotinus would begin to push for major systemization, and the Neoplatonic phase of philosophy would begin. Much of Middle Platonism would survive, but the doctrines became less eclectic, more systematically integrated, at times reinterpreted; philosophical criticism of other positions became more developed and thorough; and the questions about the relation between philosophy and religion became more complicated than they were in the days of Justin and Clement of Alexandria and others of their kind who took Christianity to be divine philosophy. But the role of Middle Platonism in early Christian life, as witnessed by people like Justin, would have significant consequences for centuries to come.