Ἐπειδὴ ἠκούσαμεν ὅτι τινὲς ἐξ ἡμῶν [ἐξελθόντες] ἐτάραξαν ὑμᾶς λόγοις ἀνασκευάζοντες τὰς ψυχὰς ὑμῶν οἷς οὐ διεστειλάμεθα
Literal translation: "Since we have heard that some out of us having gone out disturbed you with words, unsettling your souls, to whom we gave no mandate/order/command."
NAB: "Since we have heard that some of our number who went out without any mandate from us have upset you with their teachings and disturbed your peace of mind."
Recently I discussed [here] the significance of St. Paul's statement: "And how shall they preach unless they are sent?" (Romans 10:15) for understanding apostolicity. Here I wish to discuss a statement written by the Apostles and elders at the council of Jerusalem around 50 AD, and recorded by St. Luke in Acts 15:24.
If apostolic succession were merely doctrinal, then why did these disturbers need a mandate from the Apostles? Why is their lack of an Apostolic mandate even mentioned? The Apostles and elders should simply have said only that the doctrine of the disturbers was not the Apostles' doctrine. But the Apostles and elders do not say that. Instead they provide a mandate to Paul and Barnabas, Silas and Judas called Barsabbas. The "letter" mentioned in verse 23 is the authentication or proof that these men have the necessary mandate from the Apostles to teach and preach in their name, as official legates / ambassadors of the Apostles.
But if Paul and Barnabas needed an Apostolic mandate, and if the Apostles and elders show that because the disturbers mentioned in Acts 15:24 do not have an Apostolic mandate they [i.e. those disturbers] should not be listened to, then from whom did Luther, Zwingli, and Calvin receive a mandate? Or should we believe that the need for a Church mandate in order to preach and teach in the name of the Church ceased with the death of the last apostle?
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)