Brenda, please recall that atheists/agnostics make up a distinct minority of the population. There are many religious Democrats who believe in religious freedom and the freedom to not practice at all. It is already, in practice, very secular.
Freedom of/from religion is a given, but doesn’t solve the problems with religion from a platform, rather than a legal, perspective.
If we weigh all religious perspectives equally from a platform perspective, we’re giving cover to homophobia and traditional sexism. This isn’t just an issue for white Republicans. We can carve out exceptions for each instance where a community has asserted itself successfully, and then the religious are left to decide if they can stay in the party, while minorities who haven’t been able to get the party to listen to them are still left out of the platform.
These left out people are then at risk of getting treated as a fringe position. It was recently true for non-binary people and is still true for bisexuals, just two examples.
On the other side, membership in a conservative religion can lead people to compromise in favor of traditions against minorities, and genuine progressives still in traditional religions get viewed with suspicion by outsiders even when they’re staunch reformists. This last might be where Gabbard is at, but she won’t be believed.