Romans 16:18 “For they that are such serve not our Lord Jesus Christ, but their own belly; and by good words and fair speeches deceive the hearts of the simple.”
Calvin's commentary:
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.
Romans 16:18 identifies two features of false teachers:
They are not concerned for Christ’s glory but for their own pleasures.
They deceitfully creep in through alluring men with the use of “bland speech”, “flattery”, “vain praises”, and “soothing” speech—indulging others’ sins to keep their favor.
The gospel minister is instead:
A man who seeks the glory of Christ above his own pleasures.
Gospel preaching has a fitting manner. It certainly has its own courteousness and pleasantness, but it is joined with an honesty that does not flatter, spare or indulge vices, and is not designed to attach men to oneself but to the glory of Christ.