The govt already has the data on everyone in the country and what religion everyone is. Now how is and will that data be used?
The govn. does not have your religion information. If you become the subject of an FBI investigation for some reason, and they track your comings and goings, then they would find that out if you regularly go to meetings of a religious group.
Now, you can claim they know everyone’s religion, but that is a baseless claim. Also, people change their church affiliation all the time so it would be impossible to keep up with every person's religion and millions have no church affiliation, so their supposed list of over 300 million citizens would have millions of people on it with no religion listed.