Aristides

During the Antonine Era, Christians attracted the notice of Roman authors like Pliny, Tacitus, and Suetonius. The Roman authorities asked the Christians what they believed and why their beliefs should be tolerated. In western Anatolia, those beliefs were NOT tolerated; but elsewhere matters went a little better. Two Christians, Aristides and Quadratus, composed apologetics for this audience.

Quadratus didn't survive. But Aristides did: in a Syriac translation and, abridged, in a story about India (of all places).

Aristides took a tactic of pulling down other gods, rather than promote his own. He presents Christianity as a sect of Israel, which had moved into Egypt and received a Torah. This much was the default consensus of the Pagan Near East, between Ptolemy of Mendes and the second-century Bible. Christianity, for him, is a scriptural faith (ch. 2, 15). Among its writings, in ch. 2, is "the gospel"; a code of conduct is laid out ch. 15. The "gospel" is a creed, as follows:

The Christians, then, trace the beginning of their religion from Jesus the Messiah; and he is named the Son of God Most High. And it is said that God came down from heaven, and from a Hebrew virgin assumed and clothed himself with flesh; and the Son of God lived in a daughter of man. This is taught in the gospel, as it is called, which a short time ago was preached among them; and you also if you will read therein, may perceive the power which belongs to it. This Jesus, then, was born of the race of the Hebrews; and he had twelve disciples in order that the purpose of his incarnation might in time be accomplished. But he himself was pierced by the Jews, and he died and was buried; and they say that after three days he rose and ascended to heaven. Thereupon these twelve disciples went forth throughout the known parts of the world, and kept showing his greatness with all modesty and uprightness. And hence also those of the present day who believe that preaching are called Christians

This shares much with other creeds: Jesus was buried, and ascended. The Virgin Birth sounds like Matthew with a hint of John 1 and Philippians 2. The twelve disciples going out to preach to "the known parts of the world" sounds like Matthew's Great Commission.

Although... no tomb (we are sorely missing the Greek here!). And the ascension sounds immediate, without the appearances to the disciples. That Jesus was "pierced" is a Psalm 22 touch, wholly alien to Paul and to the Synoptic gospels, which all preach Christ crucified, only; the Johannine-Ignatian nails and that spear are inserted by Romans and not Jews. "After three days" is the sign of Jonah and, I think, not in our canon gospels either, where Jesus rises on the third day or even the next.

As to the code of conduct, this much is Decalogue - or even sura 17:

For they know and trust in God, the Creator of heaven and of earth, in whom and from whom are all things, to whom there is no other god as companion, from whom they received commandments which they engraved upon their minds and observe in hope and expectation of the world which is to come. Wherefore they do not commit adultery nor fornication, nor bear false witness, nor embezzle what is held in pledge, nor covet what is not theirs. They honour father and mother, and show kindness to those near to them; and whenever they are judges, they judge uprightly.

This blends into such morals as are more sectarian. The Golden Rule: whatsoever they would not that others should do unto them, they do not to others; The Luke / Matthew Sermon: And their oppressors they appease (lit: comfort) and make them their friends; they do good to their enemies. The Johannine Commandment is here they love one another; as is Mark, they call them brethren without distinction... ; for they do not call them brethren after the flesh, but brethren after the spirit and in God. Some of these actions derive from the precepts of their Messiah.

Paul's nuance in 1 Corinthians is rejected: and of the food which is consecrated to idols they do not eat, for they are pure. This is like the Clementine Homilies and the Quran, again.

Such codes of conduct look like the Didache or like the Two-Ways list in Barnabas. Examples from Jesus' life are absent. Which is not to dismiss that a narrative Gospel might lie behind some of this.