Building a community is an extensive process, and definitely not something you can do alone. You need your community to grow naturally, and the best way that can happen is by members organically engaging within.
Evangelists can simply be those members who are active that they serve as advocates who are playing a major hand in improving the overall experience of the community.
An Evangelist can be categorized as someone who's active in the community regularly. For the purpose of this example, let's say 30 days.
Set up the first filter as Active in the Last 60 days -
Now, you'll have to tag the resulting list of members as Evangelists. Click on the Update Tag option and select Add Tag. Under Map a Tag option, create a new tag 'Evangelist' or select it from the drop-down if it already exists.
Once you tagged the members, you can use this tag as a filter in the future or set up personalized campaigns with automated workflows. Here's an example of how you can do that.