On the one hand, if our information about Justin, Marcion, and Paul is even slightly correct, it seems impossible for Justin not to have at least known about the Pauline epistles, since Justin writes about Marcion (and is said by Irenaeus to have written a takedown piece on him) and Marcion is said by various fathers to have published a version of the Pauline corpus with 10 letters:
Justin Martyr, 1 Apology 58.1-2: 1 And, as we said before, the devils put forward Marcion of Pontus, who is even now teaching men to deny that God is the maker of all things in heaven and on earth, and that the Christ predicted by the prophets is His Son, and preaches another god besides the Creator of all, and likewise another son. 2 And this man many have believed, as if he alone knew the truth, and laugh at us, though they have no proof of what they say, but are carried away irrationally as lambs by a wolf, and become the prey of atheistical doctrines, and of devils.
Irenaeus, Against Heresies 4.6.2: 2 .... In his book against Marcion, Justin does well say: "I would not have believed the Lord Himself, if He had announced any other than He who is our framer, maker, and nourisher. But because the only-begotten Son came to us from the one God, who both made this world and formed us, and contains and administers all things, summing up His own handiwork in Himself, my faith towards Him is steadfast, and my love to the Father immovable, God bestowing both upon us."
But, of course, just because Justin virtually had to have known (of) the Pauline epistles does not necessarily mean that he thought of them as authoritative. Perhaps they were considered a purely Marcionite property, or at least tainted by Marcion too much to touch. Whatever the case, in his extant writings Justin never explicitly mentions the apostle Paul. (It must be remembered, though, that we do not have his book against Marcion, referenced by Irenaeus.)
Justin does, however, quote the Hebrew scriptures (in Greek) in ways reminiscent of what we find in the Pauline epistles, especially Romans and Galatians. Quite a few times they quote the same exact verses. Various scholars have used this overlap to demonstrate that Justin at various junctures was really quoting from the Pauline epistles rather than directly from the scriptures (presumably the LXX). One such scholar is Seth Ehorn in his dissertation, The Citation of Psalm 68[67].19 in Ephesians 4.8 Within the Context of Early Christian Uses of the Psalms. I take the following examples from pages 80-104.
1.
Romans 3.10-18: 10 As it is written,
"There is none righteous, not even one;
11 There is none who understands,
There is none who seeks for God;
12 All have turned aside, together they have become useless;
There is none who does good,
There is not even one.
13 Their throat is an open grave,
With their tongues they keep deceiving,
The poison of asps is under their lips;
14 Whose mouth is full of cursing and bitterness;
15 Their feet are swift to shed blood,
16 Destruction and misery are in their paths,
17 And the path of peace they have not known.
18 There is no fear of God before their eyes."
The catena in Romans draws from quite a few scriptures, and so does Justin.
Only Justin quotes the following:
16 Moreover, the Lord said, "Because the daughters of Zion are proud
And walk with heads held high and seductive eyes,
And go along with mincing steps
And tinkle the bangles on their feet,
17 Therefore the Lord will afflict the scalp of the daughters of Zion with scabs,
And the Lord will make their foreheads bare."
However, both Paul and Justin quote from the following:
1 The fool has said in his heart, "There is no God."
They are corrupt, they have committed abominable deeds;
There is no one who does good.
2 The Lord has looked down from heaven upon the sons of men
To see if there are any who understand,
Who seek after God.
3 They have all turned aside, together they have become corrupt;
There is no one who does good, not even one.
The LXX Version! This obviously incorporates Romans 3.10-18: 1 The fool has said in his heart, "There is no God." They have corrupted themselves, and become abominable in their devices; there is none that does goodness, there is not even so much as one. 2 The Lord looked down from heaven upon the sons of men, to see if there were any that understood, or sought after god. 3 They are all gone out of the way, they are together become good for nothing, there is none that does good, no not one. Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips: whose mouth is full of cursing and bitterness; their feet are swift to shed blood: destruction and misery are in their ways; and the way of peace they have not known: there is no fear of God before their eyes.
Psalm 53.1-3 (LXX 52.1-3):
1 The fool has said in his heart, "There is no God,"
They are corrupt, and have committed abominable injustice;
There is no one who does good.
2 God has looked down from heaven upon the sons of men
To see if there is anyone who understands,
Who seeks after God.
3 Every one of them has turned aside; together they have become corrupt;
There is no one who does good, not even one.
Psalm 5.9 (LXX 5.10):
9 There is nothing reliable in what they say;
Their inward part is destruction itself.
Their throat is an open grave;
They flatter with their tongue.
Psalm 140.3 (LXX 139.4):
3 They sharpen their tongues as a serpent;
Poison of a viper is under their lips.
Psalm 10.7 (LXX 9.28):
7 His mouth is full of curses and deceit and oppression;
Under his tongue is mischief and wickedness.
Isaiah 59.7-8:
7 Their feet run to evil,
And they hasten to shed innocent blood;
Their thoughts are thoughts of iniquity,
Devastation and destruction are in their highways.
8 They do not know the way of peace,
And there is no justice in their tracks;
They have made their paths crooked,
Whoever treads on them does not know peace.
Only Paul quotes from the following:
1 Transgression speaks to the ungodly within his heart;
There is no fear of God before his eyes.
Romans 3.10-18 is unattested as either present or absent in the Marcionite version of the epistle. But the selection of scriptural texts here seems too great to chalk up to coincidence.
2.
Romans 10.16-18: 16 However, they did not all heed the good news; for Isaiah says, "Lord, who has believed our report?" 17 So faith comes from hearing, and hearing by the word of Christ. 18 But I say, surely they have never heard, have they? Indeed they have; "Their voice has gone out into all the earth, and their words to the ends of the world."
Both Justin and Paul combine texts from Isaiah and from the Psalms in a similar manner:
1 Who has believed our message?
And to whom has the arm of the Lord been revealed?
Psalm 19.4:
4 Their line has gone out through all the earth,
And their utterances to the end of the world.
In them He has placed a tent for the sun....
Romans 10.16-18 is attested as absent from the Marcionite version.
3.
Romans 11.3-4: 3 "Lord, they have killed Your prophets, they have torn down Your altars, and I alone am left, and they are seeking my life." 4 But what is the divine response to him? "I have kept for Myself seven thousand men who have not bowed the knee to Baal." / 3 κύριε, τοὺς προφήτας σου ἀπέκτειναν, τὰ θυσιαστήριά σου κατέσκαψαν, κἀγὼ ὑπελείφθην μόνος καὶ ζητοῦσιν τὴν ψυχήν μου. 4 ἀλλὰ τί λέγει αὐτῷ ὁ χρηματισμός; κατέλιπον ἐμαυτῷ ἑπτακισχιλίους ἄνδρας, οἵτινες οὐκ ἔκαμψαν γόνυ τῇ Βάαλ.
Both Justin and Paul draw from the same verses about Elijah:
1 Kings 19.18: 18 "Yet I will leave 7,000 in Israel, all the knees that have not bowed to Baal and every mouth that has not kissed him." / καὶ καταλείψεις ἐν Ισραηλ ἑπτὰ χιλιάδας ἀνδρῶν πάντα γόνατα ἃ οὐκ ὤκλασαν γόνυ τῷ Βααλ καὶ πᾶν στόμα ὃ οὐ προσεκύνησεν αὐτῷ.
Granted, both verses are from the same chapter and context of 1 Kings, but still, Justin and Paul coincide against the LXX in wording.
Paul: οὐκ ἔκαμψαν γόνυ τῇ Βάαλ....
Justin: οὐκ ἔκαμψαν γόνυ τῇ Βάαλ....
Romans 11.3-4 is attested as absent from the Marcionite version.
4.
Galatians 3.10-13: 10 For as many as are of the works of the Law are under a curse; for it is written, "Cursed is everyone who does not abide by all things written in the book of the law, to perform them." 11 Now that no one is justified by the Law before God is evident; for, "The righteous man shall live by faith." 12 However, the Law is not of faith; on the contrary, "He who practices them shall live by them." 13 Christ redeemed us from the curse of the Law, having become a curse for us — for it is written, "Cursed is everyone who hangs on a tree" — 14 in order that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we would receive the promise of the Spirit through faith.
Both Justin and Paul draw upon two principal scriptural passages:
Deuteronomy 21.22-23: 22 "If a man has committed a sin worthy of death and he is put to death, and you hang him on a tree, 23 his corpse shall not hang all night on the tree, but you shall surely bury him on the same day (for he who is hanged is accursed of God), so that you do not defile your land which the Lord your God gives you as an inheritance."
Paul adds the following:
4 "Behold, as for the proud one,
His soul is not right within him;
But the righteous will live by his faith."
Leviticus 18.5: 5 So you shall keep My statutes and My judgments, by which a man may live if he does them; I am the Lord.
But again Paul and Justin agree with one another in their wording against the LXX:
Paul: Ἐπικατάρατος πᾶς ὃς οὐκ ἐμμένει πᾶσιν τοῖς γεγραμμένοις ἐν τῷ βιβλίῳ τοῦ νόμου τοῦ ποιῆσαι αὐτά.
Justin: Ἐπικατάρατος... πᾶς ὃς οὐκ ἐμμένει ἐν πᾶσιν τοῖς γεγραμμένοις ἐν τῷ βιβλίῳ τοῦ νόμου τοῦ ποιῆσαι αὐτά.
Similar phrases in Deuteronomy (LXX): τὰ γεγραμμένα ἐν τῷ βιβλίῳ τούτω (28.58); τὴν μὴ γεγραμμένην ἐν τῷ βιβλίῳ τοῦ νόμου τούτου (28.61); αἱ γεγραμμέναι ἐν τῷ βιβλίῳ τοῦ νόμου τούτου (29.19); τὰς γεγραμμένας ἐν τῷ βιβλίῳ τοῦ νόμου τούτου (29.20); τὰς γεγραμμένας ἐν τῷ βιβλίῳ τοῦ νόμου τούτου (29.26); τὰς γεγραμμένας ἐν τῷ βιβλίῳ τοῦ νόμου τούτου (30.10).
Deuteronomy: κεκατηραμένος ὑπὸ θεοῦ πᾶς κρεμάμενος ἐπὶ ξύλου.
Paul: Ἐπικατάρατος πᾶς ὁ κρεμάμενος ἐπὶ ξύλου.
Justin: Ἐπικατάρατος πᾶς ὁ κρεμάμενος ἐπὶ ξύλου.
Galatians 3.10-13 seems to be attested as present in the Marcionite version.
5.
Justin Martyr, Dialogue 87.6: I have already said, and do again say, that it had been prophesied that this would be done by Him after His ascension to heaven. It is accordingly said, "He ascended on high, He led captivity captive, He gave gifts unto the sons of men." And again, in another prophecy it is said: "And it shall come to pass after this, I will pour out My Spirit on all flesh, and on My servants, and on My handmaids, and they shall prophesy."
Ephesians 4.8: 8 Therefore it is said, "When he ascended on high he led a host of captives, and he gave gifts to men." / 8 διὸ λέγει· Ἀναβὰς εἰς ὕψος ᾐχμαλώτευσεν αἰχμαλωσίαν, ἔδωκεν δόματα τοῖς ἀνθρώποις.
Both Justin and Paul draw upon one of the Psalms here:
But once again Justin and Paul agree against the LXX in wording:
Paul: Ἀναβὰς εἰς ὕψος ᾐχμαλώτευσεν αἰχμαλωσίαν, ἔδωκεν δόματα τοῖς ἀνθρώποις.
Justin: Ἀνέβη εἰς ὕψος, ᾐχμαλώτευσεν αἰχμαλωσίαν, ἔδωκε δόματα τοῖς ἀνθρώποις.
Ephesians 4.8 is possibly attested as present in Marcion.
Ehorn takes these overlaps between Justin and Paul, over and against the LXX, as evidence that Justin knew and used the Pauline epistles. He is specifically arguing against the notion that both Justin and Paul accessed scriptural catenae; his main point against this possibility is that many of the changes made to the LXX text seem to support Paul's argument in his epistle(s). If the changes, then, are definitively Pauline, Justin's use of Paul's form of the quotation would point to his dependence upon Paul, not upon an independent catena of scriptural passages.
I personally think his argument is pretty decent in this connection. However, he does not consider a third alternative. Of the five passages above, one is unattested in the Marcionite version of Paul, while two others are attested as absent. What if, for those passages (at least), something resembling Justin's argument from the scriptures at hand was interpolated into the Marcionite version of the Pauline epistles? Granted that the changes to the LXX text are unlikely to be coincidental, what if some of them actually derive from Justin and only later made it into Paul? I am by no means committed to this solution, and would be interested in arguments against it; and it of course fails to account for the passages attested as present in the Marcionite version, presumably before Justin came along. But perhaps not all of the connections between Justin and Paul have to go in the same direction; maybe some of them Justin got from a sparser (Marcionite) edition of the epistles, while others actually flowed from Justin's text(s) into the Pauline epistles. It is something to think about, anyway. I doubt Justin himself was responsible for the interpolations, since sometimes the focus or thrust is not quite the same between Paul and Justin. But perhaps someone else, in an attempt to round out the Pauline epistles and bring them into line with the emerging Catholic orthodoxy, used Justin in order to do so.
The passages above, by the way, are not intended to exhaust the possible connections between Paul and Justin. They are merely a start, albeit probably one which hits some of the more striking correspondences.
Ben.
ETA:
Justin Martyr, Dialogue 120.5: 5 “But, since what follows indicates that the reference is to Christ — for it is, 'And He shall be the expectation of nations' (= Genesis 49.10) — I do not proceed to have a mere verbal controversy with you, as I have attempted to establish proof about Christ, not from the passages of Scripture which are not admitted by you, which I quoted from the words of Jeremiah the prophet, and Esdras, and David, but rather from those which are even now admitted by you, which, had your teachers comprehended, be well assured they would have deleted them, as they did those about the death of Isaiah, whom you sawed asunder with a wooden saw. And this was a mysterious type of Christ being about to cut your nation in two, and to raise those worthy of the honor to the everlasting kingdom along with the holy patriarchs and prophets; but He has said that He will send others to the condemnation of the unquenchable fire along with similar disobedient and impenitent men from all the nations.”
I would add that, while Justin does cite certain gospel texts, he does so almost exclusively in order to prove that the prophets had predicted Jesus Christ.