Now I beseech you, brethren, mark them who cause divisions and offenses contrary to the doctrine which you have learned; and avoid them.
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Now Paul goes on to mention the false apostles, whom he warns against throughout the epistle just as he does here as well. But he attacks their teaching without saying what it is. They were forcing believers to become Jews and thereby making the benefits of God worthless…. They compiled long genealogies and used them to support their teaching, by which they were deceiving the hearts of the simple. Commentary on Paul’s Epistles. ...
The apostle does not here say that these men caused scandals, contrary to the Scripture; but contrary to the doctrine delivered to them: this place, therefore, is an argument in favour of tradition. (Estius)
Division is the subversion of the church. Turning things upside down like this is the devil’s weapon. As long as the body is united he has no way of getting in, but harm comes from division. And where does division come from? From doctrines which are contrary to the teaching of the apostles.
1213. After indicating whom they should greet, the Apostle now shows them whom to avoid. In regard to this he does three things: first, he teaches whom they should avoid; secondly, he gives the reason [v. 18; n. 1218]; thirdly, he promises them divine help to implement this [v. 20; n. 1220]. 1214. And because those whom he wished them to avoid crept in deceptively under the cloak of piety, as it says in Mt (7:15): They come to you in sheep’s clothing, but inwardly they are ravenous wolves," he urges them to be cautious, saying: I appeal 603 to you, brethren, to observe those who create dissensions and difficulties in opposition to the doctrine you have been taught. 1215. Here, first of all, it should be noted that to observe is nothing more than to consider carefully: which, of course, is sometimes taken in a good sense and sometimes in an evil sense. It is taken in an evil sense when someone carefully considers the condition and progress of someone in order to inflict harm, as it says...