He cried, as he sat by the river Jordan: 'I baptize you with water to repentance; but He that is stronger than I shall come, whose shoes I am not worthy to bear: He shall baptize you with the Holy Ghost and with fire: whose fan is in His hand, and He will thoroughly purge His floor, and will gather the wheat into the barn; but the chaff He will burn up with unquenchable fire.' Matthew 3:11-12 And this very prophet your king Herod had shut up in prison; and when his birthday was celebrated, and the niece of the same Herod by her dancing had pleased him, he told her to ask whatever she pleased. Then the mother of the maiden instigated her to ask the head of John, who was in prison; and having asked it, [Herod] sent and ordered the head of John to be brought in on a charger. Wherefore also our Christ said, [when He was] on earth, to those who were affirming that Elijah must come before Christ: 'Elijah shall come, and restore all things; but I say unto you, that Elijah has already come, and they knew him not, but have done to him whatsoever they chose.' Matthew 17:12 And it is written, 'Then the disciples understood that He spoke to them about John the Baptist.'
The following answer by Trypho:
Trypho: This statement also seems to me paradoxical; namely, that the prophetic Spirit of God, who was in Elijah, was also in John
...is not addressed to the previous quoted Justin's passage, but to 49:3:
whose first manifestation the Spirit of God who was in Elijah preceded as herald in [the person of] John, a prophet among your nation; after whom no other prophet appeared among you.
Therefore 49:4 is an interpolation.
.
But the point is that 49:3 is also an intrrpolation!
Against the Trypho's point in 49:1-2:
Trypho: Those who affirm him to have been a man, and to have been anointed by election, and then to have become Christ, appear to me to speak more plausibly than you who hold those opinions which you express. For we all expect that Christ will be a man [born] of men, and that Elijah when he comes will anoint him. But if this man appear to be Christ, he must certainly be known as man [born] of men; but from the circumstance that Elijah has not yet come, I infer that this man is not He [the Christ].
Justin answers (4:5) :
Justin: Does not Scripture, in the book of Zechariah, Malachi 4:5 say that Elijah shall come before the great and terrible day of the Lord?
For Justin, only "the great and terrible day of the Lord" (=the SECOND Parusia) will be preceded by the Elijah redivivus. Not the FIRST Parusia.
Conclusion:
Justin didn't know that a John the Baptist had preceded, as new Elijah, the life of the Christ on the earth.