Ethic(s)

[Kaseys]

"He has questionable business ethic(s)."
"His life has been guided by (a Christian ethic)/(Christian ethics)."

When should I use the singular form or the plural form of "ethic"?

[Raymott]

"He has questionable business ethic(s)."
"His life has been guided by (a Christian ethic)/(Christian ethics)."

When should I use the singular form or the plural form of "ethic"?

The only common phrase using the singular is "work ethic". You should use the plural routinely unless/until you are sure that you mean the singular. You will almost certainly be referring to ethics, and even if you mean "ethic", the plural probably won't sound wrong.