Irenaeus's Use of 'Separate' to Describe the Heretical Gospel was Roughly Compatible with 'Secret' Mark

Irenaeus in Book Three makes clear that the Apostle Paul opposed 'separation' with respect to the godhead:

And the Apostle Paul also, saying, "For though ye have served them which are no gods; ye now know God, or rather, are known of God," has made a separation between those that were not and Him who is God ... And Paul himself says that this is true: "We know that an idol is nothing, and that there is none other God but one. For though there be that are called gods, whether in heaven or in earth; yet to us there is but one God, the Father, of whom are all things, and we through Him; and one Lord Jesus Christ, by whom are all things, and we by Him." For he has made a distinction, and separated those which are indeed called gods, but which are none, from the one God the Father, from whom are all things, and, he has confessed in the most decided manner in his own person, one Lord Jesus Christ. But in this [clause], "whether in heaven or in earth," he does not speak of the formers of the world, as these [teachers] expound it. [Adv Haer 3.6.5]

The next reference is extremely interesting as it connects the Gospel of Mark to Luke:

But Marcion, mutilating (circumcidens) that according to Luke, is proved to be a blasphemer of the only existing God, from those [passages] which he still retains. Those, again, who separate Jesus from Christ (eum Jesum separant a Christo), alleging that Christ remained impassible, but that it was Jesus who suffered, preferring the Gospel by Mark, if they read it with a love of truth, may have their errors rectified. [3.11.7] 

The in every other use of the Latin term it is clear that we are dealing with 'circumcision.'  The allusion to Marcion 'circumcizing' the gospel of Luke is clearly a clever bit of irony.  In Aramaic further more 'gospel' can be seen to be a play on the word 'flesh.'

The same idea that Peter wrongly 'separated' from the main Church is reinforced in Irenaeus as we saw it was in the Praescriptionem:

And the apostles who were with James allowed the Gentiles to act freely, yielding us up to the Spirit of God. But they themselves, while knowing the same God, continued in the ancient observances; so that even Peter, fearing also lest he might incur their reproof, although formerly eating with the Gentiles, because of the vision, and of the Spirit who had rested upon them, yet, when certain persons came from James, withdrew himself (separavit se), and did not eat with them. And Paul said that Barnabas likewise did the same thing. [3.12.15]

Irenaeus goes on to emphasize that Mark 'separated' while Luke remained 'inseparable':

But that this Luke was inseparable (inseparabilis) from Paul, and his fellow-labourer in the Gospel, he himself clearly evinces, not as a matter of boasting, but as bound to do so by the truth itself. For he says that when Barnabas, and John who was called Mark, had parted company (separatis enim) from Paul, and sailed to Cyprus, "we came to Troas;"(10) and when Paul had beheld in a dream a man of Macedonia, saying, "Come into Macedonia, Paul, and help us," "immediately," he says, "we endeavoured to go into Macedonia, understanding that the Lord had called us to preach the Gospel unto them. Therefore, sailing from Troas, we directed our ship's course towards Samothracia."[3.14.1] 

and after a long discussion of material not found in the Marcionite canon, Irenaeus concludes:

From this he shows that he was always attached to and inseparable from him (unde ostendit quod semper junctus ei et inseparabilis fuerit ab eo). [ibid]

Similarly the former associates of Peter are said to 'separate' themselves from Paul:

But again, we allege the same against those who do not recognise Paul as an apostle: that they should either reject the other words of the Gospel which we have come to know through Luke alone, and not make use of them; or else, if they do receive all these, they must necessarily admit also that testimony concerning Paul, when he (Luke) tells us that the Lord spoke at first to him from heaven: "Saul, Saul, why persecutest thou Me? I am Jesus Christ, whom thou persecutest; " and then to Ananias, saying regarding him: "Go thy way; for he is a chosen vessel unto Me, to bear My name among the Gentiles, and kings, and the children of Israel. For I will show him, from this time, how great things he must suffer for My name's sake." Those, therefore, who do not accept of him [as a teacher], who was chosen by God for this purpose, that he might boldly bear His name, as being sent to the forementioned nations, do despise the election of God, and separate themselves from the company of the apostles (et se ipsos segregant ab Apostolorum conventu.). For neither can they contend that Paul was no apostle, when he was chosen for this purpose; nor can they prove Luke guilty of falsehood, when he proclaims the truth to us with all diligence. [3.15.1]

The Valentinians act similarly according to Irenaeus in what follows:

By these words they entrap the more simple, and entice them, imitating our phraseology, that these [dupes] may listen to them the oftener; and then these are asked regarding us, how it is, that when they hold doctrines similar to ours, we, without cause, keep ourselves aloof from their company; and when they say the same things, and hold the same doctrine, we call them heretics? When they have thus, by means of questions, overthrown the faith of any, and rendered them uncontradicting hearers of their own, they describe to them in private the unspeakable mystery of their Pleroma (his separatim inenarrabile plenitudinis suse enarrant mysterium).  But they are altogether deceived, who imagine that they may learn from the Scriptural texts adduced by heretics, that [doctrine] which their words plausibly teach.  For error is plausible, and bears a resemblance to the truth, but requires to be disguised; while truth is without disguise, and therefore has been entrusted to children. [3.15.2]

We can stop here and notice that the use of 'separatim' here gets to the heart of the heretical sense of the term.  The heretics used it in the sense of 'secret' - i.e. like Secret Mark (look above).