Word for fake religious people

Consider closet ___, where ___ stands for the person’s true religion, other weltanschauung, or some relevant aspect of it. This uses closet (adjective) in the sense of:

being so in private <a closet racist>

Some examples and suggestions for possible realisations of the above:

• Arguably a good compromise between coverage, understandability, and common usage¹ is closet atheist, using atheist:

  A person who does not believe in deities.

  This works whenever the rejected religion is theistic (i.e., featuring at least on deity) and the person in question does not secretly adhere to another theistic religion.

  Here is an example of usage with a situation as the one you describe:

  

• Arguably, the most general solution would be closet apostate, using apostate:

  A person who has renounced a religion or faith.

  This works with any rejected religion and actual beliefs, even if only a specific flavour of a religion was rejected. However, it is not very common¹ and arguably less likely to be understood.

• Very commonly¹ used is closet [religion], e.g., closet christian, closet muslim, closet pastafarian, … However this only works if the person in question follows the respective specific religion in secrecy.

• Closet irreligious or closet non-religious have the advantage that they are likely to be understood and also work in the case that the rejected religion is not a theistic one.

Neither of these explicitly capture that the person in question ever believed in the religion, but this does not seem to be central to what you want to say.

^{¹ Going by Google hits.}

They are called religious pretenders or Christian (church-going) pretenders if their religion is Christianity. Pretender is

a person who professes beliefs and opinions that he or she does not hold in order to conceal his or her real feelings or motives.

[Vocabulary.com]

Actual usage: ARE YOU A CHRISTIAN PRETENDER?

This message is to all church-going pretenders out there – the ones who look heavenly on Sunday, but live like hell Monday through Saturday. You look good on the outside, but when God looks at your insides – inside your heart – it’s a dirty mess!

I’ve got bad news for you. Jesus hates religious pretense. But I’ve also got good news. He loves the pretenders! He gave His life for hypocrites like you and me. The end of being a religious pretenders is to admit our sin separates us from God; then ask Christ’s forgiveness; then believe to be right with God is your only hope for change.

I can only speak from a Protestant Christian perspective, but someone who pretends to be a Christian but clearly makes no effort to to behave like a Christian (except go to church and say the right words) could be called a hypocrite.

hypocrite noun [ C ]

someone who says they have particular moral beliefs but behaves in way that shows these are not sincere

Cambridge Dictionary

Of course, the term hypocrite can also be used in a non-religious context.

Someone who has little or no faith, but goes to church and otherwise leads a blameless life is said to be a formal Christian.

Remember that most Christians have doubts (as do members of other faiths), and these doubts can be so severe that they may wonder if they are Christians at all. I would not blame such a person for keeping quiet about it whilst still making the effort to attend church services.

Cultural

This is an adjective used to describe people who do the rigmarole for certain reasons other than the intended motivation, such as:

• Friends, family, everyone in town, etc does it
• Mama did it, Grandma did it, Great Grandma did it

Cultural Christians, for example, sit on pews every Sunday (or at least Christmas and Easter), might or might not pay attention, probably believe the very fundamental stuff (for example, there is a God and Jesus is his son) but put none of what they claim to believe into practice, potentially because they don't actually know enough about it to put it into practice.