Many like to use the moniker of this post.
Decline in numbers among the mainline churches is largely due to doctrinal drift. Older denominations may want to say it is various cultural factors of modernity, but that rings hollow.
If, as Paul said, there is not a clear call, who will rally for battle? (I Cor. 14:8). It is not very motivating in battle when the field general says it does not matter what hill his soldiers take.