Jamieson, Fausset & Brown

Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned--"which ye learned." and avoid them--The fomentors of "divisions" here referred to are probably those who were unfriendly to the truths taught in this epistle, while those who caused "offenses" were probably those referred to in Rom 14:15 as haughtily disregarding the prejudices of the weak. The direction as to both is, first, to "mark" such, lest the evil should be done ere it was fully discovered; and next, to "avoid" them (compare Th2 3:6, Th2 3:14), so as neither to bear any responsibility for their procedure, nor seem to give them the least countenance.