John Calvin Commentary Romans 16:18

"For they that are such serve not our Lord Christ, but their own belly; and by their smooth and fair speech they beguile the hearts of the innocent." — Romans 16:18 (ASV)

For those who are such, etc. He mentions an unchanging mark by which false prophets are to be distinguished from the servants of Christ; for they do not care for the glory of Christ, but seek the benefit of their stomach.

Since, however, they deceitfully crept in and, by assuming another character, concealed their own wickedness, he also pointed out, so that no one would be deceived, the tactics they adopted—that they gained favor through smooth speech.

The preachers of the gospel also have their courtesy and pleasing manner, but this is joined with honesty, so that they neither soothe people with empty praises nor flatter their vices. But impostors allure people by flattery, and spare and indulge their vices, so that they may keep them attached to themselves.

He calls those simple who are not cautious enough to avoid deceptions.