Christianity is unique, isn't it? Not really.

But the gospels…At least the gospels are unique to Christianity, aren’t they?

Well, no. 

We know that Jesus was not the only miracle-working itinerant philosopher who had students: Apollonius of Tyana was a Greek miracle-worker/itinerant philosopher who was assumed into heaven upon his death.  Apollonius was roughly contemporaneous with Jesus.  His biographer Philostratus claims he was at the court of Vardanes I of Parthia (ca 40-47 CE).  Apollonius’ territory was Cappadocia in Asia Minor.  One of the notable elements of Apollonius is the Adana Inscription.  The inscription is cut on a single large block which originally served as a lintel.  A translation of the inscription would be: 

This man, named after Apollo,
and shining forth Tyana,
extinguished the faults of men.

The tomb in Tyana (received) his body,
but in truth heaven received him
so that he might drive out the pains of men
(or:drive pains from among men). (C.P Jones, trans)

We have no similar artifact relating to Jesus.

Philostratus

In the 2^nd cen CE, the satirist Lucian of Samosata wrote an attack on Alexander of Abonoteichus, a student of a student of Apollonius’.  From this, we can infer that not only did Apollonius really exist, but that his teachings attracted followers.  Aside from the Gospels, we have no such trajectory of inheritance of tradition for teachings attributed to Jesus (largely because those teachings, such as we recognize them, are normative Judean practices based on Torah—love thy neighbor, for example, while deemed a “true Jesus saying” appears in Leviticus.)

Plihostratus attempted to reinforce the view that his subject was not a magician, but a philosopher whose teachings were consistent with Greek philosophy (in contrast to Jesus’ teachings, which were consistent with Judean texts as represented in the Torah and the Navi’im).  Eusebius (early church father and church historian) preserved some of the letters of Apollonius’ that were used by Philostratus for his biography.  Eusebius does not claim to have letters of Jesus’.  There exist other letters attributed to Apollonius which were not used by Philostratus in his biography.  No one claims to have any concrete documentation of Jesus at all. 

Dio Cassius (early church historian) remarked on Apolllonius’ death. 

In the 3^rd century CE, Empress Julia Domna promoted his teachings in Rome. Around 300, Diocletian attempted to use Apollonius’ legend as a counter to early Christian groups.

Regarding the matter of the “uniqueness” of the gospels, the texts we know from the canon are not called “euanglion  kata Markon” (for example), but simply “Kara Markon,” (according to Mark), “Kata Maththaion” (according to Matthew, and yes, there are two thetas in the name), etc.  Moreover, we know from the Nag Hammadi collection that these were not the only texts that claimed relation to the Jesus events. These were just the texts that the early church selected to form the canon.  We know from Irenaeus that the early church fathers chose four texts because “there are four humors of the body, four wind, four pillars holding up the earth, therefore there are four texts.”  (Irenaeus, Against Heresies)

Regarding the word gospel, it derives from the Greek word euangelion (verb: euangelidzo—to “good news”).  We find the verb in the place we would expect:  in Isaiah in the LXX:

Isa 40:9

 ἐπ᾽ ὄρος ὑψηλὸν ἀνάβηθι ὁ εὐαγγελιζόμενος Σιων ὕψωσον τῇ ἰσχύι τὴν φωνήν σου ὁ εὐαγγελιζόμενος Ιερουσαλημ ὑψώσατε μὴ φοβεῖσθε εἰπὸν ταῖς πόλεσιν Ιουδα ἰδοὺ ὁ θεὸς ὑμῶν

Get yourself up on a high mountain, O Zion, bearer of good news, lift up your voice with strength, O Jerusalem, bearer of good news; Lift it up, do not fear. Say to the cities of Judah, "Here is your God!"

Isa 52:7

ὡς ὥρα ἐπὶ τῶν ὀρέων ὡς πόδες εὐαγγελιζομένου ἀκοὴν εἰρήνης ὡς εὐαγγελιζόμενος ἀγαθά ὅτι ἀκουστὴν ποιήσω τὴν σωτηρίαν σου λέγων Σιων βασιλεύσει σου ὁ θεός

How lovely on the mountains Are the feet of him who brings good news, who publishes peace, who brings good tidings of good, who publishes salvation, who says to Zion, "Your God reigns."

Isa 61:1

πνεῦμα κυρίου ἐπ᾽ ἐμέ οὗ εἵνεκεν ἔχρισέν με εὐαγγελίσασθαι πτωχοῖς ἀπέσταλκέν με ἰάσασθαι τοὺς συντετριμμένους τῇ καρδίᾳ κηρύξαι αἰχμαλώτοις ἄφεσιν καὶ τυφλοῖς ἀνάβλεψιν

The Spirit of the Lord GOD is upon me, because the LORD has anointed me to bring good tidings to the afflicted; he has sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to those who are bound;

We also find it in Nahum:

Nah 1:15 

ἰδοὺ ἐπὶ τὰ ὄρη οἱ πόδες εὐαγγελιζομένου καὶ ἀπαγγέλλοντος εἰρήνην ἑόρταζε Ιουδα τὰς ἑορτάς σου ἀπόδος τὰς εὐχάς σου διότι οὐ μὴ προσθήσωσιν ἔτι τοῦ διελθεῖν διὰ σοῦ εἰς παλαίωσιν συντετέλεσται ἐξῆρται

Behold, on the mountains the feet of him who brings good tidings, who proclaims peace! Keep your feasts, O Judah, fulfil your vows, for never again shall the wicked come against you, he is utterly cut off.

However, before we get into the books of the Navi, we find it in the historical books:

2 Sam 4:10

ὅτι ὁ ἀπαγγείλας μοι ὅτι τέθνηκεν Σαουλ καὶ αὐτὸς ἦν ὡς εὐαγγελιζόμενος ἐνώπιόν μου καὶ κατέσχον αὐτὸν καὶ ἀπέκτεινα ἐν Σεκελακ ᾧ ἔδει με δοῦναι εὐαγγέλια

when one told me, saying, Behold, Saul is dead, thinking to have brought good tidings, I took hold of him, and slew him in Ziklag, which was the reward I gave him for his tidings.

2 Sam 18:27

καὶ εἶπεν ὁ σκοπός ἐγὼ ὁρῶ τὸν δρόμον τοῦ πρώτου ὡς δρόμον Αχιμαας υἱοῦ Σαδωκ καὶ εἶπεν ὁ βασιλεύς ἀνὴρ ἀγαθὸς οὗτος καί γε εἰς εὐαγγελίαν ἀγαθὴν ἐλεύσεται

And the watchman said, "I think the running of the foremost is like the running of Ahi'ma-az the son of Zadok." And the king said, "He is a good man, and comes with good tidings."

1 Kings 1:42

ἔτι αὐτοῦ λαλοῦντος καὶ ἰδοὺ Ιωναθαν υἱὸς Αβιαθαρ τοῦ ἱερέως ἦλθεν καὶ εἶπεν Αδωνιας εἴσελθε ὅτι ἀνὴρ δυνάμεως εἶ σύ καὶ ἀγαθὰ εὐαγγέλισαι

While he was still speaking, behold, Jonathan the son of Abi'athar the priest came; and Adoni'jah said, "Come in, for you are a worthy man and bring good news."

2 Kings 7:9

καὶ εἶπεν ἀνὴρ πρὸς τὸν πλησίον αὐτοῦ οὐχ οὕτως ἡμεῖς ποιοῦμεν ἡ ἡμέρα αὕτη ἡμέρα εὐαγγελίας ἐστίν καὶ ἡμεῖς σιωπῶμεν καὶ μένομεν ἕως φωτὸς τοῦ πρωὶ καὶ εὑρήσομεν ἀνομίαν καὶ νῦν δεῦρο καὶ εἰσέλθωμεν καὶ ἀναγγείλωμεν εἰς τὸν οἶκον τοῦ βασιλέως

Then they said to one another, "We are not doing right. This day is a day of good news; if we are silent and wait until the morning light, punishment will overtake us; now therefore come, let us go and tell the king's household."

Neither the word itself, euangelion, nor the subject matter (miracle-working itinerant philosopher) are unique to early Christianity.