For me, the most probable origin of Christianity (one that most on this forum refuse to consider) is Enochic Judaism, or as we call it today, Essene.
Ok I get the 'similarity' bit. A good argument can also be that Jude cites Enoch which is difficult to explain as Enoch isn't generally considered to be a canonical text. But this still doesn't answer the question. There are very few things we can absolutely be certain of in Christian antiquity. I am not sure that James and his Jewish Christian tradition is one of those things. That Paul mentioned 'James' is not certain. Tertullian's first explicit mention of Marcion in his commentary on Galatians is the confrontation with Peter:
For if such a question had arisen, others also would have been "resisted face to face" by the man who had not even spared Peter on the comparatively small matter of his doubtful conversation. But what do the Marcionites wish to have believed (on the point)? [8] For the rest, the apostle must (be permitted to) go on with his own statement, wherein he says that "a man is not justified by the works of the law, but by faith: "
Note 'Marcionites' not Marcion is mentioned here. There is a similar lack of Marcionite commentary in chapters 1 and 2 in Jerome's commentary on Galatians which was developed from Origen's lost treatise. I am not sure why a lengthy 'personal discussion' or 'history' appears in this section but it clearly contradicts the 'secret' characteristic of the Marcionite recension referenced by Tertullian at the start of Book 5.
It is noteworthy that Galatians was the first epistle of the author of Against Marcion as it was for Ephrem the Syrian and members of his Edessan community. I suggest that the 'historical' details in Galatians were added to explain who Paul was as any historical details were entirely lacking in the Marcionite epistles. The Marcionite apostle could have been anyone - even Marcion himself. The Catholic epistles were written by someone named Paul who clearly met James and a circle of apostles in Jerusalem. But were these details developed as a result of the details provided by Acts?
I require to know of Marcion the origin of his apostles even----I, who am to some degree a new disciple,4 the follower of no other master; who at the same time5 can believe nothing, except that nothing ought to be believed hastily6 (and that I may further say is hastily believed, which is believed without any examination7 of its beginning); in short, I who have the best reason possible for bringing this inquiry to a most careful solution,8 since a man is affirmed to me to be an apostle whom I do not find mentioned in the Gospel in the catalogue9 of the apostles. [2] Indeed, when I hear that this man was chosen by the Lord after He had attained His rest in heaven, I feel that a kind of improvidence is imputable to Christ, for not knowing before that this man was necessary to Him; and because He thought that he must be added to the apostolic body in the way of a fortuitous encounter10 rather than a deliberate selection; by necessity (so to speak), and not voluntary choice, although the members of the apostolate had been duly ordained, and were now dismissed to their several missions. Wherefore, O shipmaster of Pontus,11 if you have never taken on board your small craft12 any contraband goods or smuggler's cargo, if you have never thrown overboard or tampered with a freight, you are still more careful and conscientious, I doubt not, in divine things; and so I should be glad if you would inform us under what bill of lading13 you admitted the Apostle Paul on board, who ticketed him,14 what owner forwarded him,15 who handed him to you,16 that so you may land him without any misgiving,17 lest he should turn out to belong to him,18 who can substantiate his claim to him by producing all his apostolic writings.19 [3] He professes himself to be "an apostle"----to use his own, words----"not of men, nor by man, but by Jesus Christ."20 Of course, any one may make a profession concerning himself; but his profession is only rendered valid by the authority of a second person. One man signs, another countersigns;21 one man appends his seal, another registers in the public records.22 No one is at once a proposer and a seconder to himself. Besides, you have read, no doubt, that "many shall come, saying, I am Christ."23 [4] Now if any one can pretend that he is Christ, how much more might a man profess to be an apostle of Christ! But still, for my own part, I appear24 in the character of a disciple and an inquirer; that so I may even thus25 both refute your belief, who have nothing to support it, and confound your shamelessness, who make claims without possessing the means of establishing them. Let there be a Christ, let there be an apostle, although of another god; but what matter? since they are only to draw their proofs out of the Testament of the Creator. [5] Because even the book of Genesis so long ago promised me the Apostle Paul. For among the types and prophetic blessings which he pronounced over his sons, Jacob, when he turned his attention to Benjamin, exclaimed, "Benjamin shall ravin as a wolf; in the morning He shall devour the prey, and at night he shall impart nourishment."26 He foresaw that Paul would arise out of the tribe of Benjamin, a voracious wolf, devouring his prey in the morning: in order words, in the early period of his life he would devastate the Lord's sheep, as a persecutor of the churches; but in the evening he would give them nourishment, which means that in his declining years he would educate the fold of Christ, as the teacher of the Gentiles. [6] Then, again, in Saul's conduct towards David, exhibited first in violent persecution of him, and then in remorse and reparation,27 on his receiving from him good for evil, we have nothing else than an anticipation28 of Paul in Saul----belonging, too, as they did, to the same tribe----and of Jesus in David, from whom He descended according to the Virgin's genealogy.29 Should you, however, disapprove of these types,30 the Acts of the Apostles,31 at all events, have handed down to me this career of Paul, which you must not refuse to accept. Thence I demonstrate that from a persecutor he became "an apostle, not of men, neither by man; "32 thence am I led to believe the Apostle himself; thence do I find reason for rejecting your defence of him,33 and for bearing fearlessly your taunt. "Then you deny the Apostle Paul." I do not calumniate him whom I defend.34 I deny him, to compel you to the proof of him. [7] I deny him, to convince you that he is mine. If you have regard to our belief you should admit the particulars which comprise it. If you challenge us to your belief, (pray) tell us what things constitute its basis.35 Either prove the truth of what you believe, or failing in your proof, (tell us) how you believe. Else what conduct is yours,36 believing in opposition to Him from whom alone comes the proof of that which you believe?
“Finally, from so little sleeping and so much reading, his brain dried up and he went completely out of his mind.”
― Miguel de Cervantes Saavedra, Don Quixote