A) 'Acts' has the disciples (former companions of Jesus) staying in Jerusalem while Christian preaching outside Palestine were made by others (foremost Paul). That goes against the idealistic picture of the twelve, immediately after the ascension, going all over the known world in order to make converts and essentially creating the Christian world:
a) Mk16:20a (interpolation made after other gospels were known) (early 2nd century?) "And they [the disciples, right after the alleged ascension] went out and preached everywhere ..."
b) Aristides (120-130) Apology "... ascended to heaven. Thereupon these twelve disciples went forth throughout the known parts of the world ..."
c) Justin Martyr (150-160), in his 1Apology XLV "His apostles, going forth from Jerusalem, preached everywhere"
Also from Justin's works:
- 1Apology XXXIX "For from Jerusalem there went out into the world, men, twelve in number, and these illiterate, of no ability in speaking"
- 1Apology XXXIX "But the Gentiles, who had never heard anything about Christ, until the apostles set out from Jerusalem and preached concerning Him"
- Trypho LIII "For after His crucifixion, the disciples that accompanied Him were dispersed, until He rose from the dead, and persuaded them that so it had been prophesied concerning Him, that He would suffer; and being thus persuaded, they went into all the world, and taught these truths."
d) Despite attesting 'Acts' in 'Against Heresies', Irenaeus (180) wrote in his 'Demonstration apostolic':
"His disciples, the witnesses of all His good deeds, and of His teachings and His sufferings and death and resurrection, and of His ascension into heaven after His bodily resurrection----these were the apostles, who after (receiving) the power of the Holy Spirit were sent forth by Him into all the world, and wrought the calling of the Gentiles"
e) Also acknowledging 'Acts', Origen wrote (246-248), in 'Commentary of the gospel according to Matthew' X, 18:
"And the Apostles on this account left Israel and did that which had been enjoined on them by the Saviour, "Make disciples of all the nations," and, "Ye shall be My witnesses both in Jerusalem and in all Judea and Samaria, and unto the uttermost part of the earth." For they did that which had been commanded them in Judea and Jerusalem; but, since a prophet has no honour in his own country, when the Jews did not receive the Word, they went away to the Gentiles."
B) 'Acts' (as for gLuke) has several huge historical errors when comparing it with Josephus' Antiquities:
See here about the name of the high priest during the last years of Felix as prefect of Judea and here for Theudas & Judas of Galilee. Also, there are significant differences about the description of Agrippa's death and about the Egyptian false prophet.
C) 'Acts' has notable embellishments and even conflicts with the Pauline epistles on common events. For example:
a) Paul wrote he got converted because "it pleased God, who separated me from my mother's womb, and called [me] by his grace, to reveal his Son in me, that I might preach him among the heathen" (Gal 1:15-16a) while 'Acts' indicates it was the resurrected Jesus who revealed himself instead (9:4-6, 22:7-8 & 26:14-18).
b) For the same two meetings in Jerusalem, Paul wrote he did not meet the members of the Church of Jerusalem (except a few "pillars"), when 'Acts' says he did! (Gal1:18-19 <=> Ac 9:26-27; Gal 2:6,9-10 <=> Ac 15:4-21).
D) The beginning of 'Acts' does not fit well with the ending of gLuke. For example:
a) In gLuke 24:36-53, the resurrected Jesus' meeting with his disciples looks very short and immediately followed by the ascension. However, in 'Acts' (1:3-11) there are many meetings over a period of forty days.
b) In gLuke, right after the ascension, the disciples "worshipped him, and returned to Jerusalem with great joy: and were continually in the temple, praising and blessing God." (24:52-53). But in 'Acts', the same disciples and others are constantly in prayer in the upstairs "room where they were staying" and also choose a replacement for Judas (1:12-26).
Conclusion: All these points would explain why 'Acts' was not popular for a long time and rarely mentioned or quoted. That situation was still existing in the fourth century, according to John Chrysostom in his 'Homilies on the Acts of the Apostles', Homily I:
"To many persons this Book ['Acts'] is so little known, both it and its author, that they are not even aware that there is such a book in existence."
And then there is Paul (the hero of 'Acts') ...
2) About relative lack of external evidence for Paul:
Paul was not liked in some Christian circles:
Tertullian (207) wrote, in 'Against Marcion' V, I:
"I must with the best of reasons approach this inquiry with uneasiness when I find one affirmed to be an apostle, of whom in the list of the apostles in the gospel I find no trace."
"'Christ did not know beforehand that he would have need of him,"
"He [i.e., Paul] himself, says Marcion, claims to be an apostle, and that not from men nor through any man, but through Jesus Christ. Clearly any man can make claims for himself"
Also Tertullian acknowledged the heretics adopted Paul & his epistles: "the apostles of the Heretics" ('Against Marcion' III, V); which would entice Christians, like Justin Martyr, not to mention or quote Paul.
About "heretics" adopting Paul:
a) According to Hyppolitus of Rome, the Naassenes were among the first gnostics, probably starting around 110 CE.
From 'Against All Heresies", book 5, chapter 2:
"What, however, the natural use is, according to them, we shall afterwards declare. "And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly"- now the expression that which is unseemly signifies, according to these (Naasseni), the first and blessed substance, figureless, the cause of all figures to those things that are moulded into shapes,-"and receiving in themselves that recompense of their error which was meet."30 For in these words which Paul has spoken they say the entire secret of theirs, and a hidden mystery of blessed pleasure, are comprised."
It seems that very early, they made use of Paul and Romans 1:27 (in bolded italics).
Also in chapter 3 of same book:
"Paul the apostle, he says, knew of this gate, partially opening it in a mystery, and stating "that he was caught up by an angel, and ascended as far as the second and third heaven into paradise itself; and that he beheld sights and heard unspeakable words which it would not be possible for man to declare."
Here they made use of 2 Corinthians 12:3-4 (in bolded italics)
Also in same chapter:
These are, he says, what are by all called the secret mysteries, "which (also we speak), not in words taught of human wisdom, but in those taught of the Spirit, comparing spiritual things with spiritual. But the natural man receiveth not the things of the Spirit of God, for they are foolishness unto him."
Here these Naassenes made use of 1 Corinthians 2:13-14a (in bolded italics).
b) Origen said gnostic Basilides (120-140) wrote: "Indeed, the Apostle [Paul] has said, "I was once alive apart from the law," [Rom 7:9] at some time or other. That is [Paul means] , before I came into this body, I lived in the kind of body that is not subject to the law: the body of a domestic animal or a bird." (Commentary on the Epistle to the Romans 1015B)
c) Around 140-160 (before or during Justin's times), gnostic Ptolemy named Paul and paraphrased 1 Corinthians 5:7-8:
Paul the apostle shows that the Passover and the unleavened bread are images when he says, Christ our passover has been sacrificed, in order that you may be unleavened bread, not containing leaven (by leaven he here means evil), but may be a new lump.
(Epiphanius in his work Against Heresies, 33.3.1 - 33.7.10.)
d) Irenaeus confirmed in 'Against Heresies' I, III & VIII that Paul was misinterpreted by Heretics (including Valentinians).
Also, Irenaeus wrote in his 'Against Heresies' I, 26, 2 "Those who are called Ebionites agree that the world was made by God; but their opinions with respect to the Lord are similar to those of Cerinthus and Carpocrates. They use the Gospel according to Matthew [most likely a subset] only, and repudiate the Apostle Paul, maintaining that he was an apostate from the law."
Eusebius implied that Ebionism started towards the end of the 1st century.
Despite the above, and outside the writings of gnostic Marcion, Naassenes, Basilides & Ptolemy, Paul is mentioned in '1 Clement', 'Colossians', 'Acts of Apostles', 'Ephesians', '2 Thessalonians', '1 Timothy', '2 Timothy', 'Titus', '2 Peter', Papias, Ignatian 'to the Ephesians', Polycarp's epistle and 'Epistula Apostolorum'. All these texts are dated before 160 by most critical scholars. Also, these writings, with the exception of 'Acts' and the 'Epistula', are either pseudo-Pauline letters or mention Paul wrote letter(s).
Also, it is likely "Mark" knew about some Pauline epistles, as demonstrated here.
3) Did Justin Martyr quote Paul?
It appears he did, once, but indirectly, by quoting an expression from a part of gLuke already interpolated with a passage of '1 Corinthians' Last Supper.
Justin Martyr's 1 Apology LXVI
"For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, "This do ye in remembrance of Me, this is My body;" and that, after the same manner, having taken the cup and given thanks, He said, "This is My blood;" and gave it to them alone."
Let's compare that with:
"17 After taking the cup, he gave thanks and said, Take this and divide it among you.
18 For I tell you I will not drink again from the fruit of the vine until the kingdom of God comes.
19 And he took bread, gave thanks and broke it, and gave it to them, saying, This is my body given for you; do this in remembrance of me.
20 In the same way, after the supper he took the cup, saying, This cup is the new covenant in my blood, which is poured out for you.""
"do this in remembrance of me" is in both '1 Apology' and gLuke.
But it exists also in 1 Corinthians 11:23-25
"23 The Lord Jesus, ... took bread,
24 and when he had given thanks, he broke it and said, "This is my body, which is for you; do this in remembrance of me."
25 In the same way, after supper he took the cup, saying, "This cup is the new covenant in my blood; do this, whenever you drink it, in remembrance of me.""
Lk 22:19b-20 is likely a later insertion because:
a) it is lacking in Codex Bezae & some early Latin translations. And in other early manuscripts, sequence of the three clauses is changed in 22:17-20 (wine, bread, wine).
b) it duplicates the cup offering.
c) it suggests Jesus' atoning death ("which is poured out for you"), but this concept never appears again in gLuke/'Acts'.
d) it shares with 1 Co 11:24-25 expressions like "given for you", "do this in remembrance of me" & "This cup is the new covenant in my blood", not appearing in gMark & gMatthew's versions of the Last Supper.
e) "cup" and "poured out" are in nominative case, "blood" in dative. Therefore what is poured out is the cup (or its content), not the blood of Jesus, as translated in the RSV: "And likewise the cup after supper, saying, "This cup which is poured out for you is the new covenant in my blood." (note: "which is poured out for you" is located at the end of the verse in the Greek)
But in their version of the Last Supper gMark (14:24) & gMatthew (26:28) what is poured out/shed/spilled ('ekcheo') is Jesus' blood. Also in Lk 11:50 and Acts 22:20, what is shed ('ekcheo') is the blood of prophets and Stephen respectively.
It is very unlikely "Luke", an excellent Greek writer, would have made that awkward construct.
f) The expression "new covenant/testament ('diatheke')" is only used here in gLuke & Acts. And in these two books, there is no explanation about what this new covenant/testament means.
Furthermore, "Luke" wrote the (old) covenant/testament (as a promise of redemptive mercy/blessing) would be implemented by Jesus (therefore not supplanted by any new covenant/testament):
- Lk 1:68-75 "Blessed be the Lord God of Israel, for he has visited and redeemed his people,
and has raised up a horn of salvation for us in the house of his servant David, as he spoke by the mouth of his holy prophets from of old,
to perform the mercy promised to our fathers, and to remember his holy covenant,
the oath which he swore to our father Abraham,
- Acts 3:24-26 "And all the prophets who have spoken, from Samuel and those who came afterwards, also proclaimed these days.
You are the sons of the prophets and of the covenant which God gave to your fathers, saying to Abraham, 'And in your posterity shall all the families of the earth be blessed.'
God, having raised up his servant, sent him to you first, to bless you in turning every one of you from your wickedness."
So here we are: Justin Martyr, indirectly through an interpolation in gLuke, testified of the prior existence of one Pauline epistle.