Romans 16:18

For they that are such serve not our Lord Jesus Christ, but their own belly; and by their smooth and fair speech they beguile the hearts of the innocent.

The contrast here is between what the false teachers are and do on the one hand and what they pretend to be and claim on the other hand. Pretending to serve Christ, they serve themselves alone, "belly" as used here being a reference to all of the carnal and fleshly desires. They were able speakers, with a ready flow of eloquent words; and impressive rhetoric and oratory were their stock in trade. Their deceitfulness and wickedness were masked and guarded with every possible camouflage of pretended piety and devotion. Intent upon causing division as a means of drawing away disciples after themselves, these false teachers are Satan's attack forces (the shift to present tense is to focus on the problem as it still exists), not merely for the times and places known to Paul, but for all times and places, including the present now and here.

The innocent ... is Paul's reference to the naive, unsophisticated Christian, who is inclined to receive any "good speech" as the gospel truth, no matter what sacred truth may be denied by it, and never pauses to question anything, especially if the speech is a good one, and who thus unconsciously falls into the net of the false teacher.