According to Epiphanius Marcion had so altered First Thessalions (1 Thes) that he refused to comment on anything in the text:
Since Marcion has a distorted version of everything from the <First> Epistle to the Thessalonians, the fifth Epistle -- so it stands in Marcion’s canon but it is the eighth in the Apostle -- I cite nothing from it.
However, Lardner commented:
Tertullian however was of a different mind. From him it appears, that though there might be alterations made by Marcion, yet sufficient was left untouched to show the absurdity of his opinions. He says, 'it will not be unprofitable to attend to the shorter epistles; for there is a 'savouriness in few words.' He accordingly draws arguments from the 2nd, 4th, and 5th chapters of this first epistle. He says that the word "own," in the 15th verse of the second chapter, was an addition of this heretic. It is read in our present copies, but it is left out in some of the most ancient Greek MSS and by Origen, as well as Tertullian and some others.
1 Thessalonians 1:1 - 2:13
Tertullian has nothing to say regarding the beginning of 1 Thes.
1 Thessalonians 2:14-16
For ye, brethren, became followers of the churches of God which in Judaea are in Christ Jesus: for ye also have suffered like things of your own countrymen, even as they have of the Jews: [2:14] Who both killed the Lord Jesus, and their own prophets, and have persecuted us; and they please not God, and are contrary to all men: [2:15] Forbidding us to speak to the Gentiles that they might be saved, to fill up their sins alway: for the wrath is come upon them to the uttermost. [2:16]
In Tertullian's first comment on 1 Thes he writes regarding v. 2:15a:
Else he would not have burdened them with the charge of killing even the Lord, in the words, "Who both killed the Lord Jesus and their own prophets," although (the pronoun) their own be an addition of the heretics.
Although Tertullian quotes what we see in the KJV, he believes that "their own" is a heretical addition, and so worth noting. The NET Bible has the following note:
ἰδίους (idious, “their own prophets”) is found in D1 Ψ Ï sy McionT. This is obviously a secondary reading. Marcion’s influence may stand behind part of the tradition, but the Byzantine text probably added the adjective in light of its mention in v. 14 and as a clarification or interpretation of which prophets were in view.
In ‘The First New Testament: Marcion’s Scriptual Canon’ Jason D. BeDuhn writes:
Tertullian attests the reading ‘the Judeans killed their own prophets’ (occiderant Iudaei prophetas suos; interfecerunt … prophetas suos > Gk tous idious prophetas), which he claims is Marcion’s addition; but the reading is also found in Gk mss Dc, E*, K, L, Ψ, and many others, some versions and some patristic witnesses. Thus, the imaginable ideological motive for Marcion to make the change from ‘the prophets’ to ‘their (i.e., Jewish) prophets’ is beside the point, since the variant was already present in the textual tradition of Paul completely independently of Marcion (cf. Clabeaux, A Lost Edition of the Letters of Paul, 117 and n. 79).
On the persecution of Christians in Judea mentioned here, cf. Gal 6.12. Tertullian omits ‘Jesus’ following ‘the Master.’ Harnack also cites the verse from Adam 5.12, but Marcion’s text is not involved here. Pearson, ‘1 Thessalonians 2:13-16: A Deutero-Pauline Interpolation,’ has made a case for interpolation in this passage, to explain several oddities in the grammar and syntax, as well as inconsistencies with Paul’s thought elsewhere (see also Eckart, ‘Der zweite echte Brief’). While the evidence of the Apostolikon does not confirm the entire passage as an interpolation, the most severe anomalies of the text are not attested for Marcion’s text by our sources, in particular v. 15b-16. The Prologue’s reference to the persecution of the Thessalonian Christians by their fellow countrymen comes from 2.14.
Overall, the ms and patristic evidence (see also Baur, Pearson, Schmidt, Walker, Okeke, Gilliard, Bockmuehl, Horsley, Buchhold, Schippers, Dickieson, Downey, Lamp, and Schnelle) suggests that Marcion did not add "their own" to the text.
1 Thessalonians 4:3-5
Tertullian quotes from 1 Thes 4:3-5, but has no comment on how Marcion might have interpreted these verses.
1 Thessalonians 4:15-17
For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep. [4:15] For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: [4:16] Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord. [4:17]
Tertullian refers to parts of all three verses, although he may have seen some slight differences in vv. 4:15 and 4:17, as he writes:
He says that those who "remain unto the coming of Christ," along with "the dead in Christ, shall rise first," being "caught up in the clouds to meet the Lord in the air."
1 Thessalonians 5:1-11
BeDuhn writes that: “1 Thess 4.18-5.18 is unattested," and then notes:
Friedrich, ‘1 Thessalonischer 5,1-11,’ has argued that the first eleven verses of chapter 5 constitute an interpolation.”
If Marcion's version of 1 Thes did not include vv. 5:1-11 then it is possible that one reason why Epiphanius considered that: "Marcion has a distorted version of everything." is that he saw it in his own text. If so, then Tertullian's lack of comment would suggest that these verses were not present in his version of 1 Thes.
1 Thessalonians 5:19-20
Quench not the Spirit. [5:19] Despise not prophesyings. [5:20]
Tertullian refers to both verses in such a way as to make it almost certain that he saw them as we do:
Now, from whom shall I expect (the fulfilment of) all this, except from Him whom I have heard give the promise thereof? What "spirit" does he forbid us to "quench," and what "prophesyings" to "despise?"
He uses these verses to challenge Marcion's understanding, but does not suggest that Marcion made any change to the text here.
1 Thessalonians 5:23
And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ. [1 Thes 5:23]
Tertullian also uses this verse to challenge Marcion, when he says:
... (tell me) how is it that the apostle has given certain distinct names to all (our faculties), and has comprised them all in one prayer for their safety, desiring that our "spirit and soul and body may be preserved blameless unto the coming of our Lord and Saviour (Jesus) Christ?"
Tertullian adds "and saviour," and may have seen this in Marcion's text.
Summary
Although (according to Tertullian) Marcion made some minor changes to 1 Thes, it does seem very unlikely that he would actually go to the trouble of making such trivial edits, suggesting that the text of the version in the Apostolicon pre-dated Marcion.
The problem here is of course that Epiphanius reports that what he sees as Marcion's version of 1 Thes is so different to what he knows as 1 Thes that he refuses to provide any details. There are only three ways of looking at this evidence: Tertullian or Epiphanius are lying; Tertullian or Epiphanius are mistaken; or Tertullian and Epiphanius saw different versions of Marcion's Apostolicon. If we reject deliberate deception on the part of either Tertullian or Epiphanius, then could either be mistaken instead? It is not unreasonable to suggest that one could have missed a change seen by the other, but for Tertulian to have missed the "distorted version" seen by Epiphanius seems highly unlikely.
Next: Second Thessalonians