Galenus

I could not find anything in my Logos library, but I found the following on this page (I bolded your quote):

6. Reference 6

From Galen, Πλατωνικῶν διαλογων συνόψεις (=Summary of Platonic dialogues), in 8 books; from part 3.  The work is listed in De libris propriis c. 14.¹⁷

This work is lost, but a quotation is found in Arabic authors in somewhat different forms.  Hunain ibn Ishaq records that he translated a work in four parts, written by Galen in eight parts, containing summaries of works by Plato.¹⁸

The first version is found in Abu Ali Isa ibn Ishaq ibn Zura ¹⁹ (known as Ibn Zura, d. 1008 AD), On the main questions discussed between Christians and Jews. ²⁰ Walzer translation:²¹

Galen ... says at the end of his summary of Plato's Republic: "In the religious community of the followers of Christ there are most admirable people who frequently act according to perfect virtue; and this is to be seen not only in their men but in their women as well." And I see that he admires them for their virtue, and although he is a man whose position is known and whose opposition to Judaism and Christianity is manifest and clear to everybody who has studied his books and knows what he states in them, he nevertheless cannot deny the excellent qualities which the Christians display in their virtuous activities.

Ibn Abi Usaibiah (d.1270) quoting an earlier writer, `Ubayd Allāh ibn Jibrā`īl:²²

"...Evidence that Christ lived quite some time before Galen is contained in the following passage of Galen's commentary on Plato's "Republic." 'From this we may infer that the people called Christians derive their faith from signs and miracles. Also, sometimes, they show such behavior as is adopted by philosophers; for fearlessness of death and the hereafter is something we witness in them every day. The same is true of abstention from sexual intercourse. Some of them, both men and women, go their whole life without sexual intercourse. There are among them those who possess such a measure of self-control with regard to food and drink and who are so bent on justice, that they do not fall short of those who profess philosophy in truth.'

Ibn al-Qifti, History of Learned men (published after 1227 AD) has a version of the passage.  Unfortunately no English translation is available. Casiri's Latin translation is as follows:²³

Ceterum Galenum post Christum dominum natum floruisse argumento sunt illius in Commentario Libri, De Republica Platonis haec verba: Novimus gentem illam, quae Christiani nuncupantur religionem suam in parabolis et miraculis constituisse.  Cernimus praeterea illos Philosophis in morum disciplina minime cedere: caelibatum, uti et complures eorum mulieres, excolere; in cibo, potuque parsimoniam amare; in jejuniis et orationibus assiduos esse; laedere neminem: adeo ut virtutum et studio, et exercitatione Philosophos longe superaverint. 

Bar Hebraeus, Chronicum Syriacum, and the same material also in the abbreviated Arabic version, Historia Compendosia Dynastiarum.²⁵  Budge's translation of the Chronicum Syriacum:²⁴

And in his time Galen flourished. ... And he saith also in his exposition of Plato's Book of Pedon (Phaedo), 'We have seen these men who are called "Nazraye" (Nazarenes), who found their Faith upon Divine indications (or, inspirations) and miracles, and they are in no wise inferior to those who are in truth philosophers. For they love purity (or, chastity), and they are constant in Fasting, and they are zealous in avoiding the committal of wrong, and there are among them some who during the whole course of their lives never indulge in carnal intercourse. I say that this is a sign of the monastic life which became famous after the Ascension of our Lord, during the period of one hundred years'. (Budge)

Walzer states that Ibn Abi Usaibia, al-Qifti and Bar Hebraeus are all related.  He also says that Bar Hebraeus merely abbreviates al-Qifti, and the presence of "Phaedo" for "Republic" is merely one of Bar Hebraeus' "well known" mistakes.²⁶

The last version is Abu'l Fida', Universal Chronicle, book 3, chapter 3.  This covers history down to 1329 AD.  Latin translation:²⁷

Secundum El-Camil, regnante illo vixit Galenus, quamquam prima vita ejus pars in extremam Ptolemaei aetatem incidit.  Galeni tempore religion Christianorum magna jam incrementa ceperat, eorumque mentionem fecit in libro de sententiis Politiae Platonicae, his verbis: Hominem perique orationem demonstrativam continuam mente assequi nequeunt; quare indigent, ut instituantur, parabolis (narrationes dicit de praemiis et poenis in vita futura exspectandis).  Veluti nostro tempore videmus, homines illos, qui Christiani vocantur, fidem suam e parabolis petiisse.  Hi tamen interdum talia faciunt, qualia qui vere philosophantur.  Nam quod mortem contemnunt, id quidem omnes ante oculos habemus; item quod verecundia quadam ducti ab usu rerum venerearum abhorrent.  Sunt enum inter eos, et foeminae et viri, qui per totam vitam a concubitu abstinuerint; sunt etiam, qui in animis regendis coercendisque et in acerrimo honestatis studio eo progressi sint, ut nihil cedant vere philosophantibus.  Haec Galenus.

Walzer's translation:¹⁶

Most people are unable to follow any demonstrative argument consecutively; hence they need parables, and benefit from them and he (Galen) understands by parables tales of rewards and punishments in a future life -- just as now we see the people called Christians drawing their faith from parables [and miracles], and yet sometimes acting in the same way [as those who philosophize]. For their contempt of death [and of its sequel] is patent to us every day, and likewise their restraint in cohabitation. For they include not only men but also women who refrain from cohabiting all through their lives; and they also number individuals who, in self-discipline and self-control in matters of food and drink, and in their keen pursuit of justice, have attained a pitch not inferior to that of genuine philosophers.

Sprengling's translation:²⁸

According to the Kâmil [of Ibn Athîr] Galen lived in the days of this Commodus, having been born before the death of Ptolemy [literally: "and Galen lived to the time of Ptolemy"].  In his [i.e. Galen's] time the religion of the Christians had become manifest, and Galen mentions them [i.e. the Christians] in his book Remarks on the book of Plato on the Republic, where he says: "The mass of the people are not able to follow the thread of an apodictic discourse, wherefore they need allusive (enigmatic) sayings, so that they may enjoy instruction thereby (by allusive sayings he means the tales concerning rewards and punishments in the world to come). Of this sort we now see the people who are called Christians deriving their faith from such allusive sayings. Yet on their part deeds have been produced equal to the deeds of
those who are in truth philosophers. For example, that they are free from the fear of death is a fact which we all have observed; likewise their abstinence from the unlawful practice of sexual intercourse. And, indeed, there are some among them, men, and women, also, who during the whole of their natural life refrain altogether from such intercourse. And some of them have attained to such a degree of severe self-control and to such earnestness in their desire for righteousness, that they do not fall short of those who are in truth philosophers. Thus far the words of Galen.

Josh Hunt said:

I heard this quote in a sermon today. Couldn't find it anywhere in logos books. Help!

“For fearlessness of death and the hereafter is something we witness in them every day.”

Claudius Galenus (129-200 AD) - Roman Dr who examined dying Christians in the Coliseum

Not referring exactly to Your quote, but the Logos resource below includes Rodney Stark's article "EPIDEMICS, NETWORKS, AND THE RISE OF CHRISTIANITY", which also has some stuff about Galen, "the plague of Galen" (or the Antonine Plague, see also wikipedia) and the difference between the reaction of christians and pagans during the smallpox or measles epidemics in the Roman Empire during the 160's A.D.

https://www.logos.com/product/15374/semeia-56-social-networks-in-early-christian-environment-issues-and-methods-for-social-history

(It's only 5 bucks)

Somebody already pointed to this article earlier on this forum, which can also be found in the book The Rise of Christianity by Stark (not in Logos).

There's a lot of stuff about the fearless reaction of early Christians during the 160's A.D. pandemics on that article.