John Calvin on Romans 16:18

18. For they who are such, etc. He mentions an unvarying mark, by which false prophets are to be distinguished from the servants of Christ; for they have no care for the glory of Christ, but seek the benefit of their stomach. As, however, they deceitfully crept in, and by assuming another character, concealed their own wickedness, he at the same time pointed out, in order that no one might be deceived, the arts which they adopted — that they ingratiated themselves by a bland address. The preachers of the gospel have also their courtesy and their pleasing manner, but joined with honesty, so that they neither soothe men with vain praises, nor flatter their vices: but impostors allure men by flattery, and spare and indulge their vices, that they may keep them attached to themselves. He calls those simple who are not cautious enough to avoid deceptions.