This refers to those who argue and cause others to stop trusting in Jesus. AT: "who are causing believers to argue with one another and to stop having faith in God" (See: [[rc://en/ta/man/translate/figs-explicit]])
The words "they serve" are understood from the previous phrase. This can be expressed as a separate sentence. AT: "Rather, they serve their own stomach" (See: [[rc://en/ta/man/translate/figs-ellipsis]])
# but their own stomach
Here "stomach" is a metonym that refers to physical desires. Serving there stomach represents satisfying their desires. AT: "but they only want to satisfy their own selfish desires" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-metaphor]])
The words "smooth" and "flattering" mean basically the same thing. Paul is emphasizing how these people are deceiving believers. AT: "By saying things that seem to be good and true" (See: [[rc://en/ta/man/translate/figs-doublet]])
This refers to those who are simple, inexperienced, and naive. AT: "those who innocently trust them" or "those who do not know these teachers are fooling them"