The letter is, to us, tediously rambling with long stretches telling us how wonderful God is. But look closely at those praises of God and one sees that they are all about the "Creator" God. This is one of the indicators (only one of them) that the letter was a response to the Marcionite propaganda. Marcion claimed the Creator God was not a very good god. 1 Clement in opposition says he was the greatest.
Another oddity about the letter is the curiously brief reference to Peter and Paul. So brief, some have suggested the line was a later interpolation. But the brief mention can also be understood as the first tentative effort to "establish" the notion that not only had Paul come to Rome (there was no doubt about that) but that Peter was associated with him and also visited Rome, and the two were "companions together in the Lord" so to speak, being martyred there. The point is that the two were "inseparable companions". Why introduce Peter to Rome? Was not Paul being there enough? Again, think Marcion: by adding Peter to Paul here we have another potentially anti-Marcionite blow. The two cannot be separated as they were in Marcion's polemics.
Now Eusebius tells us that around 170 CE one of the points of the letter of 1 Clement was to associate Peter with Paul. Paul had visited Corinth and Rome, but the letter wants readers to know that Paul and Peter were "as one" even in death.
One of the issues 1 Clement raises is that of the necessity to believe in the resurrection of the body and the coming of God or Jesus back to earth. Those were the same concerns of Justin. Justin even said those who claimed to be Christians yet who did not believe these things were "false Christians". He is talking about Marcionites (among others, perhaps).
Turmel has much more, but the point is that 1 Clement share the same doctrines and the same enemies. DeepL'ing a passage from page 72:
Clement further, as mentioned above, writes long praises of God which begin to take on relevance when he adds that the same God will judge the wicked, those who think they can defy him.... Clement is talking about "a special kind of sinner", those who have departed from the Creator God and Judge.The rebels of Corinth whom Clement tries to bring to resignation belong to the same party as the heretics whose theories Justin denounces with indignation and to whom he refuses the title of Christian. Like them, they reject the resurrection of the body and the earthly kingdom of Christ, but like them they admit that the soul of the Christian enters heaven as soon as it leaves the body.
What is the relevance of all of this to the problems at Corinth? It seems like a meaningless detour in a letter otherwise professing to solve a serious power dispute and factionalism in Corinth. But it does become very relevant in a situation where the divisions in Corinth are initiated by the introduction of Marcionism.
Turmel's thesis, then, is that the revolt in Corinth was caused by inroads of Marcionism. The priests who had been deposed were the anti-Marcionites. To support this thesis Turmel shows how it makes sense of a number of otherwise obscure passages in 1 Clement, or if not obscure, certainly apparently otherwise quite irrelevant.
1 Clement, then, is an apology for the God of the OT; it explains, contrary to Marcionism, that the OT God does not lead people into sin but seeks to save them. All those rambling passages become powerful punches when read against the background of Marcionism.
IF we accept the above thesis, then the letter of 1 Clement cannot be older than 150 CE. As Turmel points out, one must go on to test the hypothesis through external controls, or "history" (pp. 81f). (In other words, doing some real historical research instead of naive proof-texting.) Turmel's section zeroing in on the question of the date is on pages 81 to 84. I'll translate again:
I have highlighted a sentence in the second last paragraph that should alert us to Turmel's scholarly approach, his refusal to over-state the importance of certain kinds of evidence even if it would mean pushing his argument all the harder. A good example, he is.The letter of Polycarp makes extensive use of the Roman Clement. If it were of about 112 as is claimed, the letter of Clement would have to be placed at the latest in the first years of the second century. But Polycarp's letter, which openly denounces Marcion, cannot be earlier than 150, and can even be dated later than 160, since the old chronology, to which we must return, places Polycarp's death in 166 (1). In these conditions the letter of Polycarp only proves that the letter of Clement of Rome is prior to 165.
The letter written, around 170, by Denys of Corinth to the Romans ends with these words (Eusebius, IV, 23, 11):
So in Corinth, around 170, there were two letters from Rome: one very recent, to which Denys is answering here, and another older one. It was also known that the older letter had been written by Clement. This last piece of information could only have been given to the Corinthians by the bearers of the letter, since the letter itself is silent on this point. He was one of those who are quickly forgotten, because they become indifferent. Yet Denys did not forget him. He knows that the "first" letter was written by Clement. This persistence of the memory would be strange, if it were a letter written three quarters of a century earlier. It is understandable, on the contrary, if we suppose that Denys himself saw the bearers of the letter and was informed by them about its writing. In short, Denys of Corinth invites us to place the letter of Roman Clement in his lifetime, for example fifteen or twenty years before the letter which he himself wrote to the Romans.On this holy Sunday which we have celebrated today, we have read your letter which we shall always continue to read for our instruction, as we read the first one written to us by Clement.
Let us move on to Hermas. It is admitted today that he was the brother of Pius, and that his book is placed around 140 or even later. Now Hermas tells us (vision II, 4. 3) that he was charged to give a booklet to Clement whose function is to correspond with the cities of the exterior. This information has thrown the critics into confusion, for they all firmly believe that Clement's letter is from around 96. They had to explain how a man who, around 96, wrote a letter in the name of the church in Rome, could be contemporary with another man whose book appeared in 140 or perhaps later. According to some critics, the book of Hermas is the work of two Christian authors, one of whom lived in the last years of the first century, while the other belongs to the middle of the following century. According to a second opinion, Hermas presented himself as the contemporary of Clement, who lived around 96, for the sole purpose of backdating his book and artificially setting it back half a century before the time of its appearance. Others say that Hermas, who completed his book around 140, had begun it as early as 110, and that Clement was still living at that date. Finally, according to others, the Clement of whom Hermas speaks and who is in charge of corresponding with the cities of the exterior, would differ from the Clement who wrote the letter to the Corinthians around 96. All these alleged solutions, each more improbable than the other, prove that the problem is insoluble for the critics. It vanishes when we are certain that Clement's letter to the Corinthians was written around the middle of the second century.
The witnesses we have questioned so far leave us free to place Clement in the middle of the second century, if we wish. Some of them even invite us strongly to do so. Irenaeus sounds a different note. He tells us (III, 3, that the church of Rome, founded and constituted by the apostles Peter and Paul, was governed after them by Linus, of whom Paul speaks in his epistles, then by Anacletus, then by Clement; that this Clement had seen the apostles, had lived with them, and that he used an imposing letter to calm the troubles which had arisen in the church of Corinth. A contemporary of the apostles Peter and Paul obviously could not prolong his life beyond the reign of Trajan. To place the letter of Clement in the middle of the second century, it is necessary first of all to discard the testimony of Irenaeus. Can we?
Irenaeus, to whom we owe information of capital importance, is like all his contemporaries, of a credulity of fantasy. He is a witness who is worth what his sources are worth. After having spoken of Clement, he makes parade before us successors Evaristus, Alexander, Xystus, Telesphorus, Hyginus, Anicetus, Soter and finally Eleutherus. In short, he serves us what is called the Episcopal List of Rome. Let us see what this list is worth.
I won't address Turmel's discussion on Irenaeus's list that dates Clement very early. Maybe in another post I can set out his reasons for thinking that the list was created by Hegesippus and how he went about it, etc. Or you can read it via https://www.jstor.org/stable/23664428