The GDPR means that people can process and store your data only on a legal basis, and that they have to document their processes and their compliance within the rules.
They can process data
- based on your (informed and revocable) consent,
- in accordance with a contract with you,
- if required by law,
- to safeguard the vital interests of others (likely not applicable),
- as a government agency (almost certainly not applicable),
- and finally in a balance of legitimate interests.
The first bullet point is the easiest for them and for you, until you ask them to delete. Then they have to use another justification. A forum may argue that keeping discussion threads intact outweighs the desire to have them deleted. Here it may matter that data about religious beliefs is one of the specially protected categories. This might shift the balance of interests.