First, a doctrine simply falls from mention. Over time, it is simply never discussed or presented from the pulpit. Most congregants do not even miss the mention of the doctrine. Those who do become fewer over time. The doctrine is not so much denied as ignored and kept at a distance. Yes, it is admitted, that doctrine has been believed by Christians, but it is no longer a necessary matter of emphasis.
Actually I think there is a prior step, which is for some to begin to complain that we do too much preaching about it, and do so in excessive and lurid and manipulative terms. It's the beginning of the downgrade.