He did not have a New Testament to fall back on. He knew the teachings of the Apostles and had the Magisterium of the Church. He was a pagan philosopher who converted to Christianity. Martyr is not his last name, rather it is how he died—by having his head cut off for his belief in Christ.
Here is what Justin Martyr wrote about the Eucharist: What is it; Who can partake of it; and How Must we Live to Partake…
“And on the day called Sunday, all who live in the cities or in the country gather together to one place and the memoirs of the apostles or the writings of the prophets are read, as long as time permits; then, when the reader has ceased, the president [priest] verbally instructs, and exhorts to the imitation of these good things.
Then we all rise together and pray, and, as we before said, when our prayer is ended bread and wine are brought, and the president in like manner offers prayers and thanksgivings, according to his ability, and the people assent, saying Amen, and there is a distribution to each and a participation of that over which thanks have been given [Eucharist elements] and to those who are absent a portion is sent by the deacons.”
“And this food is called among us Εὐχαριστία [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined.
For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Savior, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh.”
