[color=rgba(0, 0, 0, 0.87)]But lest some should, without reason, and for the perversion of what we teach, maintain that we say that Christ was born one hundred and fifty years ago under Cyrenius, and subsequently, in the time of Pontius Pilate, taught what we say He taught; and should cry out against us as though all men who were born before Him were irresponsible — let us anticipate and solve the difficulty. 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 Misael, and Elias, and many others whose actions and names we now decline to recount, because we knowit would be tedious. So that even they who lived before Christ, and lived without reason, were wicked and hostile to Christ, and slew those who lived reasonably. But who, through the power of the Word, according to the will of God the Father and Lord of all, He was born of a virgin as a man, and was named Jesus, and was crucified, and died, and rose again, and ascended into heaven, an intelligent man will be able to comprehend from what has been already so largely said. And we, since the proof of this subject is less needful now, will pass for the present to the proof of those things which are urgent.[/color]

St Justin was saying that there were God-fearing men who lived among the atheists (i.e. Plato/Socrates) and among the barbarians (i.e. Abraham and the 3 men in the fiery furnace).  In the Old Testament, to be saved, you had to follow the 10 commandments, believe in the one True God and have faith in the coming redeemer.  There is evidence that Plato/Socrates rejected the Greek's polytheism and believed many Jєωιѕн doctrines.  Socrates is said to have been murdered for his preaching contrary to the Greek pagans.
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https://torreybookblog.wordpress.com/2013/03/18/justins-apology-for-plato/
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The point is that St Justin makes the case that Plato/Socrates were not pagans but believed many things about the coming Christ and had good will.  St Thomas speaks highly of Aristotle in this regard too.  Maybe they made it to Limbo?

But there was no Catholic Church to join in the time of Plato/Socrates/Abraham.  There was only the Jєωιѕн religion and salvation depended on faith and the 10 commandments more than joining the Jєωιѕн religion (I could be wrong here, but since the Jєωιѕн religion was centered in Jerusalem and not the entire world, as is the Catholic religion's goal, then salvation in the Old Testament was less strict and more based on belief and the natural law since not every man could go to Jerusalem and live there).  The article I linked is saying that St Justin believed that Christ (the logos) revealed Himself to Plato/Socrates just like He did to Abraham.  This is why St Justin called them "Christians", so the article says.

Plato/Socrates were known to preach contrary to the entire Greek way of life.  They believed in natural virtue and (presumably) tried to live according to the natural law.  As the saying goes, "grace builds on nature".  God rewards those who live according to their conscience (as is everyone's duty).
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A study of history shows that the Chinese were not open to the Faith, even when missionaries finally went there after the middle ages.  Even the great St Francis Xavier could barely make a few converts in Japan after years of toil.  And Japan learned all their pagan ideas from China, so the two cultures were very similar.  St Francis noted that once he learned about the Japanese culture, he said that the Chinese would have to be converted first, before Japan would follow.
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A study of native American history shows a similar thing.  Most native tribes were utterly pagan and snake-worshipping.  Their culture was based on war and death.  Even the most anti-Catholic historians readily admit this.  Much like the Aztec Indians who offered infants in sacrifice to their gods, the American Indians were ruthless.  They didn't follow the natural law, so they would not be open to spiritual graces.
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The few exceptions of the American tribes show that when the missionaries arrived, they found that these Indians already knew about the one, true God and they recognized the "black robes" (as they called the priests) because God had sent them saints to preach the Faith.  There is also ample historical evidence that the Russians, Celts and other countries traveled to America long before Columbus first arrived.  They brought the Faith with them as well.

How do we know that God didn't send saints miraculously to preach in China or Japan to these few, just as He did to the American Indians?  Our Faith says we must believe God would do so.

Exactly.  That is what St Justin is saying, that Christ revealed Himself to Plato, just as Abraham was told about the Redeemer.  Just as God miraculously sent St Peter to Cornelius in Scripture.  Just as Blessed Mary of Agreda bilocated to preach to the American Indians.  Just like St Thomas the Apostle was miraculously transported from India to see the Assumption of Our Lady with the rest of the Apostles.  Just as St Padre Pio was transported to hear a dying man's confession who was on some remote snowy mountain.
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Our Faith tells us, and so does Scripture, that "God's hand is not shortened to save..."  (Isaiah)

re: “Today you shall be with me in paradise” Was the thief on the cross baptised? Was he saved?

The good thief was under the old covenant. He did not go directly to heaven, he went to the same place as Adam, Eve, Abraham, and the Holy Innocents all of which were not under the obligation of being baptized to be saved. Heaven was not opened till the Resurrection of Our Lord.

From The Catechism of Trent: Baptism Made Obligatory After Christ's Resurrection

The second period to be distinguished, that is, the time when the law of Baptism was made, also admits of no doubt. Holy writers are unanimous in saying that after the Resurrection of our Lord, when He gave to His Apostles the command to go and teach all nations: baptising them in the name of the Father, and of the Son, and of the Holy Ghost, the law of Baptism became obligatory on all who were to be saved.