For they that are such serve not our Lord Jesus Christ, but their own belly (cf. Philippians 3:18, Philippians 3:19). Had St. Paul thought these people sincere though mistaken, he would doubtless have treated them with the tenderness he shows towards the weak brethren.
But he regards them as self-interested, and of the flesh; and against such disturbers of the Church's peace he is, here as elsewhere, indignant (el. Galatians 1:7, Galatians 1:8; Galatians 2:4; Galatians 3:1; Galatians 5:11.
12). In speaking of them as serving, or being slaves to, their own belly, it cannot be concluded certainly that he attributed to them habits of sensuality. He may only mean that it is the gratification of the lower part of their nature that they have in view; and there may be allusion to the motive of such persons being the desire of eating and drinking at the cost of the Churches.
In 'The Teaching of the Twelve Apostles' (alluded to under Romans 12:6, seq.) the desire to live without working at the cost of the Church is set down as one of the marks of a false apostle or a false prophet.
And by good words and fair speeches deceive the hearts of the simple (rather, innocent, or harmless. So the word ἄκακος is translated in Hebrews 7:26. It is different from ἀκέραιος in Hebrews 7:19, though the Authorized Version makes no difference).
For your obedience is come abroad unto all men. This is apparently adduced as a reason for his exhorting them to beware of those seducers, with a confidence that they will not be seduced by them, Romans 16:19 being thus dependent on Romans 16:17.
I am glad therefore on your behalf: but yet I would have you wise unto that which is good, but simple ( ἀκεραίους) concerning evil. And the God of peace shall bruise Satan under your feet shortly. The grace of our Lord Jesus Christ be with you all.
Amen.