Someone that uses the Christian religion to hide their lack of faith, immoral actions, and societal misdeeds on the guise that Jesus died for their sins. These people will sometimes attend church every sunday, or maybe even 'just' occasionally (and sometimes never at all) just to be looped into a large group in order to feel as though their shallow beliefs will be backed up because of sheer numbers.
Sunday Christians will also take anything said about their religion personally, and sometimes as an attack on "their" god. This gives them a false cause and a reason to feel righteous about themselves.
Sunday Christians are very rarely moral or honorable people. They assume that they are good people solely based on their religion, rather than their actions. Sunday Christians are frauds to the religion's bedrock of actual do-gooders.
"It doesn't matter if I ______, Jesus died for my sins, so I'll go to heaven."
Insert one of the following: Lie, cheat, steal, blasphem, murder, commit adultery, prejudge, am a racist, etc.
The term "Sunday Christian" is not usable in the example sentence.
A person who acts like a Christian on a Sunday but does not the rest of the week.
"See that woman up there, shes a real Sunday Christian"