An enemy is a powerful tool to unite a community.
An enemy can be an idea, an upcoming event, or a rival group. But it's best if the enemy is a real person.
Who stands for everything your community stands against? Who has the most influence in preventing the progress of your members and their ideals? Is it someone within the community ecosystem that is holding you back or the leader of someone outside that system?
For a Barista community, it might be the leader or outspoken member of a coffee chain that's hurting independent retails, maybe Howard Schultz. Or it might be someone that sells rival products that aren't as good. Perhaps it's the leader of instant coffee drinks?
For a parenting community, it might be Supernanny or a popular author of books advocating a way of parenting your community disagrees with. For liberals, it might be Rush Limbaugh or Glenn Beck.
Identify the enemy. Criticise their views in your material and consistently challenge their contributions. This isn't a call to create a mob who are willing to make personal attacks on an individual. It's a call to unite a community against an enemy that symbolizes what your community stands against. It's a call to openly challenge the individuals views where possible. It's a call to create discussions about their views. It's a call to create content which criticises them. It's a call to prove how and why they are wrong.
Don't get personal. Don't get overly aggressive. Attack their views and contributions, not the person.
Aside, there is a danger that if you don't identify an enemy, you become the enemy. You can become seen as an opressor for the community. You're the one holding them back.