"Some respected style guides such as The Chicago Manual of Style recommend the more modern addition of an s when forming the singular regular possessive of a noun ending in "s" but specifically state that adding simply an apostrophe (eg. Jesus') is also correct.
The Elements of Style and the Canadian Press Stylebook hold that the s is mandatory with only two exceptions: classical and Biblical proper names (e.g. "Jesus' teachings", "Augustus' guards") and common phrases that do not take the s (e.g. "for goodness' sake"). In all other cases, it is incorrect to omit the s."